diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 600e628..b8972de 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,6 +21,9 @@ jobs:
           - '3.0'
           - 3.1
           - 3.2
+          - 3.3
+          - 3.4
+          - 4.0
           - jruby
 
     steps:
diff --git a/mutations.gemspec b/mutations.gemspec
index a1b90be..3dfc2d7 100644
--- a/mutations.gemspec
+++ b/mutations.gemspec
@@ -14,6 +14,6 @@ Gem::Specification.new do |s|
   s.require_path = 'lib'
 
   s.add_dependency "activesupport"
-  s.add_development_dependency 'minitest', '~> 4'
+  s.add_development_dependency 'minitest'
   s.add_development_dependency 'rake'
 end
diff --git a/spec/additional_filter_spec.rb b/spec/additional_filter_spec.rb
index 1efb48f..31d1e05 100644
--- a/spec/additional_filter_spec.rb
+++ b/spec/additional_filter_spec.rb
@@ -39,7 +39,7 @@ def execute
     it "should recognize additional filters" do
       outcome = TestCommandUsingAdditionalFilters.run(:first_name => "John", :last_name => "Doe")
       assert outcome.success?
-      assert_equal nil, outcome.errors
+      assert_nil outcome.errors
     end
 
     class TestCommandUsingAdditionalFiltersInHashes < Mutations::Command
@@ -60,7 +60,7 @@ def execute
       )
 
       assert outcome.success?
-      assert_equal nil, outcome.errors
+      assert_nil outcome.errors
     end
 
     class TestCommandUsingAdditionalFiltersInArrays < Mutations::Command
@@ -81,7 +81,7 @@ def execute
       )
 
       assert outcome.success?
-      assert_equal nil, outcome.errors
+      assert_nil outcome.errors
     end
 
     module Mutations
diff --git a/spec/array_filter_spec.rb b/spec/array_filter_spec.rb
index 74edb06..6380f04 100644
--- a/spec/array_filter_spec.rb
+++ b/spec/array_filter_spec.rb
@@ -9,7 +9,7 @@ class SimpleModel; end
     f = Mutations::ArrayFilter.new(:arr)
     filtered, errors = f.filter([1])
     assert_equal [1], filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "considers non-arrays to be invalid" do
@@ -23,7 +23,7 @@ class SimpleModel; end
   it "considers nil to be invalid" do
     f = Mutations::ArrayFilter.new(:arr, :nils => false)
     filtered, errors = f.filter(nil)
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :nils, errors
   end
 
@@ -31,20 +31,20 @@ class SimpleModel; end
     f = Mutations::ArrayFilter.new(:arr, :nils => true)
     _filtered, errors = f.filter(nil)
     _filtered, errors = f.filter(nil)
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "lets you specify a class, and has valid elements" do
     f = Mutations::ArrayFilter.new(:arr, :class => Integer)
     filtered, errors = f.filter([1,2,3])
-    assert_equal nil, errors
+    assert_nil errors
     assert_equal [1,2,3], filtered
   end
 
   it "lets you specify a class as a string, and has valid elements" do
     f = Mutations::ArrayFilter.new(:arr, :class => 'Integer')
     filtered, errors = f.filter([1,2,3])
-    assert_equal nil, errors
+    assert_nil errors
     assert_equal [1,2,3], filtered
   end
 
@@ -125,7 +125,7 @@ class SimpleModel; end
 
     filtered, errors = f.filter([true, false, "1"])
     assert_equal [true, false, true], filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "lets you pass model in arrays" do
@@ -133,7 +133,7 @@ class SimpleModel; end
 
     filtered, errors = f.filter(["hey"])
     assert_equal ["hey"], filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "lets you pass hashes in arrays" do
@@ -153,8 +153,8 @@ class SimpleModel; end
     filtered, errors = f.filter([{:foo => "f", :bar => 3, :baz => true}, {:foo => "f", :bar => 3}, {:foo => "f"}])
     assert_equal [{:foo=>"f", :bar=>3, :baz=>true}, {:foo=>"f", :bar=>3}, {:foo=>"f"}], filtered
 
-    assert_equal nil, errors[0]
-    assert_equal nil, errors[1]
+    assert_nil errors[0]
+    assert_nil errors[1]
     assert_equal({"bar"=>:required}, errors[2].symbolic)
   end
 
@@ -167,7 +167,7 @@ class SimpleModel; end
 
     filtered, errors = f.filter([["h", "e"], ["l"], [], ["lo"]])
     assert_equal filtered, [["h", "e"], ["l"], [], ["lo"]]
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "handles errors for arrays of arrays" do
@@ -189,14 +189,14 @@ class SimpleModel; end
     end
     filtered, errors = f.filter([1, "2", "three", "4", 5, [6]])
     assert_equal [1,2,4,5], filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "considers long arrays to be valid" do
     f = Mutations::ArrayFilter.new(:arr, :min_length => 2)
     filtered, errors = f.filter([1, 2])
     assert_equal [1, 2], filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "considers short arrays to be invalid" do
@@ -210,7 +210,7 @@ class SimpleModel; end
     f = Mutations::ArrayFilter.new(:arr, :max_length => 20)
     filtered, errors = f.filter([1])
     assert_equal [1], filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
   
   it "sets a max_length for the array of 2 with 5 as invalid" do
@@ -229,7 +229,7 @@ class SimpleModel; end
     m = SimpleModel.new
     filtered, errors = f.filter([m])
     assert_equal [m], filtered
-    assert_equal nil, errors
+    assert_nil errors
 
     Object.send(:remove_const, 'SimpleModel')
     class SimpleModel; end
@@ -251,7 +251,7 @@ class SimpleModel; end
     m = SimpleModel.new
     filtered, errors = f.filter([m])
     assert_equal [m], filtered
-    assert_equal nil, errors
+    assert_nil errors
 
     Object.send(:remove_const, 'SimpleModel')
     class SimpleModel; end
@@ -259,7 +259,7 @@ class SimpleModel; end
     m = SimpleModel.new
     filtered, errors = f.filter([m])
     assert_equal [m], filtered
-    assert_equal nil, errors
+    assert_nil errors
 
     Mutations.cache_constants = was
   end
diff --git a/spec/boolean_filter_spec.rb b/spec/boolean_filter_spec.rb
index 0049e0d..dd3ac38 100644
--- a/spec/boolean_filter_spec.rb
+++ b/spec/boolean_filter_spec.rb
@@ -6,11 +6,11 @@
     f = Mutations::BooleanFilter.new
     filtered, errors = f.filter(true)
     assert_equal true, filtered
-    assert_equal nil, errors
+    assert_nil errors
 
     filtered, errors = f.filter(false)
     assert_equal false, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "considers non-booleans to be invalid" do
@@ -24,15 +24,15 @@
   it "considers nil to be invalid" do
     f = Mutations::BooleanFilter.new(:nils => false)
     filtered, errors = f.filter(nil)
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :nils, errors
   end
 
   it "considers nil to be valid" do
     f = Mutations::BooleanFilter.new(:nils => true)
     filtered, errors = f.filter(nil)
-    assert_equal nil, filtered
-    assert_equal nil, errors
+    assert_nil filtered
+    assert_nil errors
   end
 
   it "considers certain strings to be valid booleans" do
@@ -40,7 +40,7 @@
     [["true", true], ["TRUE", true], ["TrUe", true], ["1", true], ["false", false], ["FALSE", false], ["FalSe", false], ["0", false], [0, false], [1, true]].each do |(str, v)|
       filtered, errors = f.filter(str)
       assert_equal v, filtered
-      assert_equal nil, errors
+      assert_nil errors
     end
   end
   
diff --git a/spec/command_spec.rb b/spec/command_spec.rb
index 881622b..6cfa9a6 100644
--- a/spec/command_spec.rb
+++ b/spec/command_spec.rb
@@ -9,7 +9,7 @@
 
       assert outcome.success?
       assert_equal({:name => "John", :email => "john@gmail.com", :amount => 5}.stringify_keys, outcome.result)
-      assert_equal nil, outcome.errors
+      assert_nil outcome.errors
     end
 
     it "should filter out spurious params" do
@@ -17,7 +17,7 @@
 
       assert outcome.success?
       assert_equal({:name => "John", :email => "john@gmail.com", :amount => 5}.stringify_keys, outcome.result)
-      assert_equal nil, outcome.errors
+      assert_nil outcome.errors
     end
 
     it "should discover errors in inputs" do
@@ -70,7 +70,7 @@
 
       assert !outcome.success?
       assert_equal :max_length, outcome.errors.symbolic[:name]
-      assert_equal nil, outcome.errors.symbolic[:email]
+      assert_nil outcome.errors.symbolic[:email]
     end
 
     it "should merge multiple hashes" do
diff --git a/spec/date_filter_spec.rb b/spec/date_filter_spec.rb
index a71f87a..2792fc1 100644
--- a/spec/date_filter_spec.rb
+++ b/spec/date_filter_spec.rb
@@ -6,7 +6,7 @@
     f = Mutations::DateFilter.new
     filtered, errors = f.filter(date)
     assert_equal date, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
   
   it "takes a DateTime object" do
@@ -14,7 +14,7 @@
     f = Mutations::DateFilter.new
     filtered, errors = f.filter(date)
     assert_equal date, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
   
   it "takes a Time object and converts it to a date" do
@@ -22,7 +22,7 @@
     f = Mutations::DateFilter.new
     filtered, errors = f.filter(time)
     assert_equal time.to_date, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "checks if the given date is after a certain date" do
@@ -32,7 +32,7 @@
     filtered, errors = f.filter(date)
 
     assert_equal date, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "gives errors when the given date is before the after date" do
@@ -41,7 +41,7 @@
     f = Mutations::DateFilter.new(:after => after_date)
     filtered, errors = f.filter(date)
 
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :after, errors
   end
 
@@ -52,7 +52,7 @@
     filtered, errors = f.filter(date)
 
     assert_equal date, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "gives errors when the given date is after the before date" do
@@ -61,7 +61,7 @@
     f = Mutations::DateFilter.new(:before => before_date)
     filtered, errors = f.filter(date)
 
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :before, errors
   end
 
@@ -73,7 +73,7 @@
     filtered, errors = f.filter(date)
 
     assert_equal date, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "should be able to parse a D-M-Y string to a date" do
@@ -83,7 +83,7 @@
     filtered, errors = f.filter(date_string)
 
     assert_equal date, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "should be able to parse a Y-M-D string to a date" do
@@ -93,7 +93,7 @@
     filtered, errors = f.filter(date_string)
 
     assert_equal date, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "should be able to handle date formatting" do
@@ -103,21 +103,21 @@
     filtered, errors = f.filter(date_string)
 
     assert_equal date, filtered
-    assert_equal nil, errors
+    assert_nil errors
 
     date_string = "1, 2, 2000"
     f = Mutations::DateFilter.new(:format => '%m, %d, %Y')
     filtered, errors = f.filter(date_string)
 
     assert_equal date, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "considers nil to be invalid" do
     f = Mutations::DateFilter.new
     filtered, errors = f.filter(nil)
 
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :nils, errors
   end
 
@@ -132,8 +132,8 @@
     f = Mutations::DateFilter.new(:nils => true)
     filtered, errors = f.filter(nil)
 
-    assert_equal nil, filtered
-    assert_equal nil, errors
+    assert_nil filtered
+    assert_nil errors
   end
 
   it "considers empty strings to be nil if empty_is_nil option is used" do
@@ -145,8 +145,8 @@
   it "returns empty strings as nil if empty_is_nil option is used" do
     f = Mutations::DateFilter.new(:empty_is_nil => true, :nils => true)
     filtered, errors = f.filter("")
-    assert_equal nil, filtered
-    assert_equal nil, errors
+    assert_nil filtered
+    assert_nil errors
   end
 
   it "doesn't allow non-existing dates" do
@@ -154,7 +154,7 @@
     f = Mutations::DateFilter.new
     filtered, errors = f.filter(date_string)
 
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :date, errors
   end
 end
diff --git a/spec/duck_filter_spec.rb b/spec/duck_filter_spec.rb
index ab2d99e..046c9b5 100644
--- a/spec/duck_filter_spec.rb
+++ b/spec/duck_filter_spec.rb
@@ -6,11 +6,11 @@
     f = Mutations::DuckFilter.new(:methods => [:length])
     filtered, errors = f.filter("test")
     assert_equal "test", filtered
-    assert_equal nil, errors
+    assert_nil errors
 
     filtered, errors = f.filter([1, 2])
     assert_equal [1, 2], filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "doesn't allow objects that respond to a single specified method" do
@@ -27,15 +27,15 @@
   it "considers nil to be invalid" do
     f = Mutations::DuckFilter.new(:nils => false)
     filtered, errors = f.filter(nil)
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :nils, errors
   end
 
   it "considers nil to be valid" do
     f = Mutations::DuckFilter.new(:nils => true)
     filtered, errors = f.filter(nil)
-    assert_equal nil, filtered
-    assert_equal nil, errors
+    assert_nil filtered
+    assert_nil errors
   end
 
   it "Allows anything if no methods are specified" do
@@ -43,7 +43,7 @@
     [true, "hi", 1, [1, 2, 3]].each do |v|
       filtered, errors = f.filter(v)
       assert_equal v, filtered
-      assert_equal nil, errors
+      assert_nil errors
     end
   end
 end
diff --git a/spec/file_filter_spec.rb b/spec/file_filter_spec.rb
index ce071c3..e318f72 100644
--- a/spec/file_filter_spec.rb
+++ b/spec/file_filter_spec.rb
@@ -13,7 +13,7 @@ class UploadedStringIO < StringIO
     f = Mutations::FileFilter.new
     filtered, errors = f.filter(file)
     assert_equal file, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows files - stringio class" do
@@ -21,7 +21,7 @@ class UploadedStringIO < StringIO
     f = Mutations::FileFilter.new
     filtered, errors = f.filter(file)
     assert_equal file, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows files - tempfile" do
@@ -29,7 +29,7 @@ class UploadedStringIO < StringIO
     f = Mutations::FileFilter.new
     filtered, errors = f.filter(file)
     assert_equal file, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "doesn't allow non-files" do
@@ -46,15 +46,15 @@ class UploadedStringIO < StringIO
   it "considers nil to be invalid" do
     f = Mutations::FileFilter.new(:nils => false)
     filtered, errors = f.filter(nil)
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :nils, errors
   end
 
   it "considers nil to be valid" do
     f = Mutations::FileFilter.new(:nils => true)
     filtered, errors = f.filter(nil)
-    assert_equal nil, filtered
-    assert_equal nil, errors
+    assert_nil filtered
+    assert_nil errors
   end
   
   it "considers empty strings to be empty" do
@@ -68,7 +68,7 @@ class UploadedStringIO < StringIO
     f = Mutations::FileFilter.new(:size => 4)
     filtered, errors = f.filter(file)
     assert_equal file, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "shouldn't allow big files" do
@@ -84,7 +84,7 @@ class UploadedStringIO < StringIO
     f = Mutations::FileFilter.new(:upload => true)
     filtered, errors = f.filter(file)
     assert_equal file, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "should require extra methods if uploaded file: deny" do
diff --git a/spec/float_filter_spec.rb b/spec/float_filter_spec.rb
index 0bbfd96..2f47eb0 100644
--- a/spec/float_filter_spec.rb
+++ b/spec/float_filter_spec.rb
@@ -6,42 +6,42 @@
     f = Mutations::FloatFilter.new
     filtered, errors = f.filter(3.1415926)
     assert_equal 3.1415926, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows strings that start with a digit" do
     f = Mutations::FloatFilter.new
     filtered, errors = f.filter("3")
     assert_equal 3.0, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows string representation of float" do
     f = Mutations::FloatFilter.new
     filtered, errors = f.filter("3.14")
     assert_equal 3.14, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows string representation of float without a number before dot" do
     f = Mutations::FloatFilter.new
     filtered, errors = f.filter(".14")
     assert_equal 0.14, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows negative strings" do
     f = Mutations::FloatFilter.new
     filtered, errors = f.filter("-.14")
     assert_equal(-0.14, filtered)
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows strings with a positive sign" do
     f = Mutations::FloatFilter.new
     filtered, errors = f.filter("+.14")
     assert_equal 0.14, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "doesnt't allow other strings, nor does it allow random objects or symbols" do
@@ -55,15 +55,15 @@
   it "considers nil to be invalid" do
     f = Mutations::FloatFilter.new(:nils => false)
     filtered, errors = f.filter(nil)
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :nils, errors
   end
 
   it "considers nil to be valid" do
     f = Mutations::FloatFilter.new(:nils => true)
     filtered, errors = f.filter(nil)
-    assert_equal nil, filtered
-    assert_equal nil, errors
+    assert_nil filtered
+    assert_nil errors
   end
   
   it "considers empty strings to be empty" do
@@ -83,7 +83,7 @@
     f = Mutations::FloatFilter.new(:min => 10)
     filtered, errors = f.filter(31)
     assert_equal 31, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "considers high numbers invalid" do
@@ -97,7 +97,7 @@
     f = Mutations::FloatFilter.new(:max => 10)
     filtered, errors = f.filter(3)
     assert_equal 3, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
 end
diff --git a/spec/hash_filter_spec.rb b/spec/hash_filter_spec.rb
index c4402dc..235d9bf 100644
--- a/spec/hash_filter_spec.rb
+++ b/spec/hash_filter_spec.rb
@@ -9,7 +9,7 @@
     end
     filtered, errors = hf.filter(:foo => "bar")
     assert_equal({"foo" => "bar"}, filtered)
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it 'disallows non-hashes' do
@@ -29,7 +29,7 @@ def input.to_hash
       string :foo
     end
     _filtered, errors = hf.filter(input)
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows wildcards in hashes" do
@@ -38,7 +38,7 @@ def input.to_hash
     end
     filtered, errors = hf.filter(:foo => "bar", :baz => "ban")
     assert_equal({"foo" => "bar", "baz" => "ban"}, filtered)
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows floats in hashes" do
@@ -47,7 +47,7 @@ def input.to_hash
     end
     filtered, errors = hf.filter(:foo => 3.14)
     assert_equal({"foo" => 3.14}, filtered)
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows ducks in hashes" do
@@ -56,7 +56,7 @@ def input.to_hash
     end
     filtered, errors = hf.filter(:foo => "123")
     assert_equal({"foo" => "123"}, filtered)
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows dates in hashes" do
@@ -65,7 +65,7 @@ def input.to_hash
     end
     filtered, errors = hf.filter(:foo => "1-1-2000")
     assert_equal Date.new(2000, 1, 1), filtered[:foo]
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows files in hashes" do
@@ -75,7 +75,7 @@ def input.to_hash
     end
     filtered, errors = hf.filter(:foo => sio)
     assert_equal({"foo" => sio}, filtered)
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "doesn't allow wildcards in hashes" do
@@ -93,7 +93,7 @@ def input.to_hash
     end
     filtered, errors = hf.filter(:foo => "bar", :baz => "4")
     assert_equal({"foo" => "bar", "baz" => 4}, filtered)
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "doesn't allow a mix of specific keys and then wildcards -- should raise errors appropriately" do
@@ -118,7 +118,7 @@ def input.to_hash
 
       filtered, errors = hf.filter(:foo => "bar")
       assert_equal({"foo" => "bar"}, filtered)
-      assert_equal nil, errors
+      assert_nil errors
     end
 
     it "bar is optional -- it works if nil is passed" do
@@ -133,7 +133,7 @@ def input.to_hash
 
       filtered, errors = hf.filter(:foo => "bar", :bar => nil)
       assert_equal({"foo" => "bar"}, filtered)
-      assert_equal nil, errors
+      assert_nil errors
     end
 
     it "bar is optional -- it works if nil is passed and nils are allowed" do
@@ -148,7 +148,7 @@ def input.to_hash
 
       filtered, errors = hf.filter(:foo => "bar", :bar => nil)
       assert_equal({"foo" => "bar", "bar" => nil}, filtered)
-      assert_equal nil, errors
+      assert_nil errors
     end
   end
 
@@ -165,7 +165,7 @@ def input.to_hash
 
       filtered, errors = hf.filter(:foo => "bar", :bar => "")
       assert_equal({"foo" => "bar"}, filtered)
-      assert_equal nil, errors
+      assert_nil errors
     end
     
     it "bar is optional -- discards empty if it needs to be stripped" do
@@ -180,7 +180,7 @@ def input.to_hash
 
       filtered, errors = hf.filter(:foo => "bar", :bar => " ")
       assert_equal({"foo" => "bar"}, filtered)
-      assert_equal nil, errors
+      assert_nil errors
     end
     
     it "bar is optional -- don't discard empty if it's spaces but stripping is off" do
@@ -195,7 +195,7 @@ def input.to_hash
 
       filtered, errors = hf.filter(:foo => "bar", :bar => " ")
       assert_equal({"foo" => "bar", "bar" => " "}, filtered)
-      assert_equal nil, errors
+      assert_nil errors
     end
 
     it "bar is optional -- errors if discard_empty is false and value is blank" do
@@ -224,7 +224,7 @@ def input.to_hash
 
       filtered, errors = hf.filter(:foo => "bar", :bar => "", :baz => "\t")
       assert_equal({"foo" => "bar"}, filtered)
-      assert_equal nil, errors
+      assert_nil errors
     end
   end
 
@@ -241,7 +241,7 @@ def input.to_hash
 
       filtered, errors = hf.filter(:foo => "bar", :bar => "baz")
       assert_equal({"foo" => "bar"}, filtered)
-      assert_equal nil, errors
+      assert_nil errors
     end
 
     it "should discard invalid optional values for wildcards" do
@@ -256,7 +256,7 @@ def input.to_hash
 
       filtered, errors = hf.filter(:foo => "bar", :bar => "baz", :wat => 1)
       assert_equal({"foo" => "bar", "wat" => 1}, filtered)
-      assert_equal nil, errors
+      assert_nil errors
     end
 
 
diff --git a/spec/integer_filter_spec.rb b/spec/integer_filter_spec.rb
index 5b690c2..39e0161 100644
--- a/spec/integer_filter_spec.rb
+++ b/spec/integer_filter_spec.rb
@@ -6,21 +6,21 @@
     f = Mutations::IntegerFilter.new
     filtered, errors = f.filter(3)
     assert_equal 3, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows strings that start with a digit" do
     f = Mutations::IntegerFilter.new
     filtered, errors = f.filter("3")
     assert_equal 3, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows negative strings" do
     f = Mutations::IntegerFilter.new
     filtered, errors = f.filter("-3")
     assert_equal(-3, filtered)
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "doesnt't allow other strings, nor does it allow random objects or symbols" do
@@ -34,15 +34,15 @@
   it "considers nil to be invalid" do
     f = Mutations::IntegerFilter.new(:nils => false)
     filtered, errors = f.filter(nil)
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :nils, errors
   end
 
   it "considers nil to be valid" do
     f = Mutations::IntegerFilter.new(:nils => true)
     filtered, errors = f.filter(nil)
-    assert_equal nil, filtered
-    assert_equal nil, errors
+    assert_nil filtered
+    assert_nil errors
   end
   
   it "considers empty strings to be empty" do
@@ -60,8 +60,8 @@
   it "returns empty strings as nil if empty_is_nil option is used" do
     f = Mutations::IntegerFilter.new(:empty_is_nil => true, :nils => true)
     filtered, errors = f.filter("")
-    assert_equal nil, filtered
-    assert_equal nil, errors
+    assert_nil filtered
+    assert_nil errors
   end
 
   it "considers low numbers invalid" do
@@ -75,7 +75,7 @@
     f = Mutations::IntegerFilter.new(:min => 10)
     filtered, errors = f.filter(31)
     assert_equal 31, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "considers high numbers invalid" do
@@ -89,7 +89,7 @@
     f = Mutations::IntegerFilter.new(:max => 10)
     filtered, errors = f.filter(3)
     assert_equal 3, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "considers not matching numbers to be invalid" do
diff --git a/spec/model_filter_spec.rb b/spec/model_filter_spec.rb
index 7fa1d5b..e23c463 100644
--- a/spec/model_filter_spec.rb
+++ b/spec/model_filter_spec.rb
@@ -20,7 +20,7 @@ def new_record?
     m = SimpleModel.new
     filtered, errors = f.filter(m)
     assert_equal m, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   # it "disallows different types of models" do
@@ -50,15 +50,15 @@ def new_record?
   it "considers nil to be invalid" do
     f = Mutations::ModelFilter.new(:simple_model, :nils => false)
     filtered, errors = f.filter(nil)
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :nils, errors
   end
 
   it "considers nil to be valid" do
     f = Mutations::ModelFilter.new(:simple_model, :nils => true)
     filtered, errors = f.filter(nil)
-    assert_equal nil, filtered
-    assert_equal nil, errors
+    assert_nil filtered
+    assert_nil errors
   end
 
   it "will not re-constantize if cache_constants is true" do
@@ -68,7 +68,7 @@ def new_record?
     m = SimpleModel.new
     filtered, errors = f.filter(m)
     assert_equal m, filtered
-    assert_equal nil, errors
+    assert_nil errors
 
     Object.send(:remove_const, 'SimpleModel')
 
@@ -89,7 +89,7 @@ class SimpleModel; end
     m = SimpleModel.new
     filtered, errors = f.filter(m)
     assert_equal m, filtered
-    assert_equal nil, errors
+    assert_nil errors
     
     Object.send(:remove_const, 'SimpleModel')
     
@@ -98,7 +98,7 @@ class SimpleModel; end
     m = SimpleModel.new
     filtered, errors = f.filter(m)
     assert_equal m, filtered
-    assert_equal nil, errors
+    assert_nil errors
     
     Mutations.cache_constants = was
   end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 31a38b3..bfb660f 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,4 +1,3 @@
-require 'minitest/unit'
 require 'minitest/autorun'
 require 'pp'
 
diff --git a/spec/string_filter_spec.rb b/spec/string_filter_spec.rb
index 8c7e6fd..eb6f142 100644
--- a/spec/string_filter_spec.rb
+++ b/spec/string_filter_spec.rb
@@ -8,21 +8,21 @@
     sf = Mutations::StringFilter.new
     filtered, errors = sf.filter("hello")
     assert_equal "hello", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows symbols" do
     sf = Mutations::StringFilter.new
     filtered, errors = sf.filter(:hello)
     assert_equal "hello", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows numbers" do
     sf = Mutations::StringFilter.new
     filtered, errors = sf.filter(1)
     assert_equal "1", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "disallows non-string" do
@@ -37,28 +37,28 @@
     sf = Mutations::StringFilter.new(:strip => true)
     filtered, errors = sf.filter(" hello ")
     assert_equal "hello", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "doesn't strip" do
     sf = Mutations::StringFilter.new(:strip => false)
     filtered, errors = sf.filter(" hello ")
     assert_equal " hello ", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "considers nil to be invalid" do
     sf = Mutations::StringFilter.new(:nils => false)
     filtered, errors = sf.filter(nil)
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :nils, errors
   end
 
   it "considers nil to be valid" do
     sf = Mutations::StringFilter.new(:nils => true)
     filtered, errors = sf.filter(nil)
-    assert_equal nil, filtered
-    assert_equal nil, errors
+    assert_nil filtered
+    assert_nil errors
   end
 
   it "considers empty strings to be nil if empty_is_nil option is used" do
@@ -70,8 +70,8 @@
   it "returns empty strings as nil if empty_is_nil option is used" do
     f = Mutations::StringFilter.new(:empty_is_nil => true, :nils => true)
     filtered, errors = f.filter("")
-    assert_equal nil, filtered
-    assert_equal nil, errors
+    assert_nil filtered
+    assert_nil errors
   end
 
   it "considers empty strings to be invalid" do
@@ -85,7 +85,7 @@
     sf = Mutations::StringFilter.new(:empty => true)
     filtered, errors = sf.filter("")
     assert_equal "", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "considers stripped strings that are empty to be invalid" do
@@ -98,14 +98,14 @@
   it "considers stripped strings that are blank to be nil if empty_is_nil option is used" do
     sf = Mutations::StringFilter.new(:strip => true, :empty_is_nil => true, :nils => true)
     filtered, errors = sf.filter("   ")
-    assert_equal nil, filtered
-    assert_equal nil, errors
+    assert_nil filtered
+    assert_nil errors
   end
 
   it "considers stripped strings that are blank to be invalid if empty_is_nil option is used" do
     sf = Mutations::StringFilter.new(:strip => true, :empty_is_nil => true)
     filtered, errors = sf.filter("   ")
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :nils, errors
   end
 
@@ -127,7 +127,7 @@
     sf = Mutations::StringFilter.new(:max_length => 5)
     filtered, errors = sf.filter("12345")
     assert_equal "12345", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "considers short strings to be invalid" do
@@ -141,7 +141,7 @@
     sf = Mutations::StringFilter.new(:min_length => 5)
     filtered, errors = sf.filter("12345")
     assert_equal "12345", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "considers bad matches to be invalid" do
@@ -155,7 +155,7 @@
     sf = Mutations::StringFilter.new(:matches => /aaa/)
     filtered, errors = sf.filter("baaab")
     assert_equal "baaab", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "considers non-inclusion to be invalid" do
@@ -169,42 +169,42 @@
     sf = Mutations::StringFilter.new(:in => %w(red blue green))
     filtered, errors = sf.filter("red")
     assert_equal "red", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "converts symbols to strings" do
     sf = Mutations::StringFilter.new(:strict => false)
     filtered, errors = sf.filter(:my_sym)
     assert_equal "my_sym", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "converts integers to strings" do
     sf = Mutations::StringFilter.new(:strict => false)
     filtered, errors = sf.filter(1)
     assert_equal "1", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "converts bigdecimals to strings" do
     sf = Mutations::StringFilter.new(:strict => false)
     filtered, errors = sf.filter(BigDecimal("0.0001"))
     assert_equal("0.1E-3", filtered.upcase)
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "converts floats to strings" do
     sf = Mutations::StringFilter.new(:strict => false)
     filtered, errors = sf.filter(0.0001)
     assert_equal "0.0001", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "converts booleans to strings" do
     sf = Mutations::StringFilter.new(:strict => false)
     filtered, errors = sf.filter(true)
     assert_equal "true", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "disallows symbols" do
@@ -247,28 +247,28 @@
     sf = Mutations::StringFilter.new(:allow_control_characters => false)
     filtered, errors = sf.filter("Hello\u0000\u0000World!")
     assert_equal "Hello World!", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "doesn't remove control characters" do
     sf = Mutations::StringFilter.new(:allow_control_characters => true)
     filtered, errors = sf.filter("Hello\u0000\u0000World!")
     assert_equal "Hello\u0000\u0000World!", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "doesn't remove tabs, spaces and line breaks" do
     sf = Mutations::StringFilter.new(:allow_control_characters => false)
     filtered, errors = sf.filter("Hello,\tWorld !\r\nNew Line")
     assert_equal "Hello,\tWorld !\r\nNew Line", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "doesn't remove emoji" do
     sf = Mutations::StringFilter.new(:allow_control_characters => false)
     filtered, errors = sf.filter("😂🙂🙃🤣🤩🥰🥱")
     assert_equal "😂🙂🙃🤣🤩🥰🥱", filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
 end
diff --git a/spec/symbol_filter_spec.rb b/spec/symbol_filter_spec.rb
index ccc9377..a6b8936 100644
--- a/spec/symbol_filter_spec.rb
+++ b/spec/symbol_filter_spec.rb
@@ -6,14 +6,14 @@
     sf = Mutations::SymbolFilter.new
     filtered, errors = sf.filter("hello")
     assert_equal :hello, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "allows symbols" do
     sf = Mutations::SymbolFilter.new
     filtered, errors = sf.filter(:hello)
     assert_equal :hello, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "doesn't allow non-symbols" do
@@ -27,15 +27,15 @@
   it "considers nil to be invalid" do
     sf = Mutations::SymbolFilter.new(:nils => false)
     filtered, errors = sf.filter(nil)
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :nils, errors
   end
 
   it "considers nil to be valid" do
     sf = Mutations::SymbolFilter.new(:nils => true)
     filtered, errors = sf.filter(nil)
-    assert_equal nil, filtered
-    assert_equal nil, errors
+    assert_nil filtered
+    assert_nil errors
   end
 
   it "considers non-inclusion to be invalid" do
@@ -49,7 +49,7 @@
     sf = Mutations::SymbolFilter.new(:in => [:red, :blue, :green])
     filtered, errors = sf.filter(:red)
     assert_equal :red, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
 end
diff --git a/spec/time_filter_spec.rb b/spec/time_filter_spec.rb
index 38d3888..b3e19ab 100644
--- a/spec/time_filter_spec.rb
+++ b/spec/time_filter_spec.rb
@@ -7,7 +7,7 @@
     f = Mutations::TimeFilter.new
     filtered, errors = f.filter(time)
     assert_equal time, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "takes a Date object and converts it to a time" do
@@ -15,7 +15,7 @@
     f = Mutations::TimeFilter.new
     filtered, errors = f.filter(date)
     assert_equal date.to_time, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "takes a DateTime object and converts it to a time" do
@@ -23,7 +23,7 @@
     f = Mutations::TimeFilter.new
     filtered, errors = f.filter(date)
     assert_equal date.to_time, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "checks if the given time is after a certain time" do
@@ -31,14 +31,14 @@
     f = Mutations::TimeFilter.new(:after => time - 1)
     filtered, errors = f.filter(time)
     assert_equal time, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "gives errors when the given time is before the after time" do
     time = Time.now
     f = Mutations::TimeFilter.new(:after => time + 1)
     filtered, errors = f.filter(time)
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :after, errors
   end
 
@@ -47,14 +47,14 @@
     f = Mutations::TimeFilter.new(:before => time + 1)
     filtered, errors = f.filter(time)
     assert_equal time, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "gives errors when the given time is after the before time" do
     time = Time.now
     f = Mutations::TimeFilter.new(:before => time - 1)
     filtered, errors = f.filter(time)
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :before, errors
   end
 
@@ -63,7 +63,7 @@
     f = Mutations::TimeFilter.new(:after => time - 1, :before => time + 1)
     filtered, errors = f.filter(time)
     assert_equal time, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "should be able to parse a D-M-Y string to a time" do
@@ -72,7 +72,7 @@
     f = Mutations::TimeFilter.new
     filtered, errors = f.filter(date_string)
     assert_equal date.to_time, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "should be able to parse a Y-M-D string to a time" do
@@ -81,7 +81,7 @@
     f = Mutations::TimeFilter.new
     filtered, errors = f.filter(date_string)
     assert_equal date.to_time, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "should be able to handle time formatting" do
@@ -90,27 +90,27 @@
     f = Mutations::TimeFilter.new(:format => '%Y-%m-%d %H:%M:%S')
     filtered, errors = f.filter(time_string)
     assert_equal time, filtered
-    assert_equal nil, errors
+    assert_nil errors
 
     time_string = "1, 2, 2000, 121314"
     f = Mutations::TimeFilter.new(:format => '%m, %d, %Y, %H%M%S')
     filtered, errors = f.filter(time_string)
     assert_equal time, filtered
-    assert_equal nil, errors
+    assert_nil errors
   end
 
   it "considers nil to be invalid" do
     f = Mutations::TimeFilter.new
     filtered, errors = f.filter(nil)
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :nils, errors
   end
 
   it "allows the use of nil when specified" do
     f = Mutations::TimeFilter.new(:nils => true)
     filtered, errors = f.filter(nil)
-    assert_equal nil, filtered
-    assert_equal nil, errors
+    assert_nil filtered
+    assert_nil errors
   end
 
   it "considers empty strings to be empty" do
@@ -124,7 +124,7 @@
     invalid_time_string = "1, 20, 2013 25:13"
     f = Mutations::TimeFilter.new
     filtered, errors = f.filter(invalid_time_string)
-    assert_equal nil, filtered
+    assert_nil filtered
     assert_equal :time, errors
   end
 end