diff --git a/include/UniTensor.hpp b/include/UniTensor.hpp
index 0ece48653..fcee9f48f 100644
--- a/include/UniTensor.hpp
+++ b/include/UniTensor.hpp
@@ -606,7 +606,14 @@ namespace cytnx {
     void print_diagram(const bool &bond_info = false) const;
     void print_blocks(const bool &full_info = true) const;
     void print_block(const cytnx_int64 &idx, const bool &full_info = true) const;
-    Tensor get_block(const cytnx_uint64 &idx = 0) const { return this->_block.clone(); }
+    Tensor get_block() const { return this->_block.clone(); }
+    Tensor get_block(const cytnx_uint64 &idx) const {
+      cytnx_error_msg(idx != 0,
+                      "[ERROR][DenseUniTensor] Dense tensor has only one block, block number %llu "
+                      "invalid. Use get_block(0).\n",
+                      (unsigned long long)idx);
+      return this->_block.clone();
+    }
 
     Tensor get_block(const std::vector<cytnx_int64> &qnum, const bool &force) const {
       cytnx_error_msg(
@@ -631,9 +638,23 @@ namespace cytnx {
     }
 
     // return a share view of block, this only work for non-symm tensor.
-    Tensor &get_block_(const cytnx_uint64 &idx = 0) { return this->_block; }
+    Tensor &get_block_() { return this->_block; }
+    Tensor &get_block_(const cytnx_uint64 &idx) {
+      cytnx_error_msg(idx != 0,
+                      "[ERROR][DenseUniTensor] Dense tensor has only one block, block number %llu "
+                      "invalid. Use get_block_(0).\n",
+                      (unsigned long long)idx);
+      return this->_block;
+    }
     // return a share view of block, this only work for non-symm tensor.
-    const Tensor &get_block_(const cytnx_uint64 &idx = 0) const { return this->_block; }
+    const Tensor &get_block_() const { return this->_block; }
+    const Tensor &get_block_(const cytnx_uint64 &idx) const {
+      cytnx_error_msg(idx != 0,
+                      "[ERROR][DenseUniTensor] Dense tensor has only one block, block number %llu "
+                      "invalid. Use get_block_(0).\n",
+                      (unsigned long long)idx);
+      return this->_block;
+    }
 
     cytnx_uint64 Nblocks() const { return 1; };
     std::vector<Tensor> get_blocks() const {
@@ -655,8 +676,8 @@ namespace cytnx {
       return this->_interface_block;  // this will not share memory!!
     }
 
-    void put_block(const Tensor &in, const cytnx_uint64 &idx = 0) {
-      // We don't check the dtype for DenseUniTensor, since it'll be more convinent to change
+    void put_block(const Tensor &in) {
+      // We don't check the dtype for DenseUniTensor, since it'll be more convenient to change
       // DenseUniTensor's dtype
 
       // cytnx_error_msg(in.dtype() != this->dtype(),
@@ -681,9 +702,16 @@ namespace cytnx {
         this->_block = in.clone();
       }
     }
+    void put_block(const Tensor &in, const cytnx_uint64 &idx) {
+      cytnx_error_msg(idx != 0,
+                      "[ERROR][DenseUniTensor] Dense tensor has only one block, block number %llu "
+                      "invalid. Use put_block(0).\n",
+                      (unsigned long long)idx);
+      put_block(in);
+    }
     // share view of the block
-    void put_block_(Tensor &in, const cytnx_uint64 &idx = 0) {
-      // We don't check the dtype for DenseUniTensor, since it'll be more convinent to change
+    void put_block_(Tensor &in) {
+      // We don't check the dtype for DenseUniTensor, since it'll be more convenient to change
       // DenseUniTensor's dtype
 
       // cytnx_error_msg(in.dtype() != this->dtype(),
@@ -708,6 +736,13 @@ namespace cytnx {
         this->_block = in;
       }
     }
+    void put_block_(Tensor &in, const cytnx_uint64 &idx) {
+      cytnx_error_msg(idx != 0,
+                      "[ERROR][DenseUniTensor] Dense tensor has only one block, block number %llu "
+                      "invalid. Use put_block_(0).\n",
+                      (unsigned long long)idx);
+      put_block_(in);
+    }
 
     void put_block(const Tensor &in, const std::vector<cytnx_int64> &qnum, const bool &force) {
       cytnx_error_msg(
diff --git a/tests/DenseUniTensor_test.cpp b/tests/DenseUniTensor_test.cpp
index f9d7d9774..18b5bfe32 100644
--- a/tests/DenseUniTensor_test.cpp
+++ b/tests/DenseUniTensor_test.cpp
@@ -1266,11 +1266,9 @@ TEST_F(DenseUniTensorTest, get_block) {
 /*=====test info=====
 describe:test get_block out of range
 ====================*/
-#if FAIL_CASE_OPEN
 TEST_F(DenseUniTensorTest, get_block_out_of_range) {
   EXPECT_THROW(utzero345.get_block(3), std::logic_error);
 }
-#endif
 
 /*=====test info=====
 describe:test get_block, diagonal
@@ -1386,11 +1384,9 @@ TEST_F(DenseUniTensorTest, get_block__uninit) {
 /*=====test info=====
 describe:test get_block out of range
 ====================*/
-#if FAIL_CASE_OPEN
 TEST_F(DenseUniTensorTest, get_block__out_of_range) {
   EXPECT_THROW(utzero345.get_block_(3), std::logic_error);
 }
-#endif
 
 TEST_F(DenseUniTensorTest, get_blocks) { EXPECT_THROW(utzero345.get_blocks(), std::logic_error); }
 
@@ -1496,7 +1492,6 @@ TEST_F(DenseUniTensorTest, put_block_rank_mismatch) {
 /*=====test info=====
 describe:test put_block_, out of index
 ====================*/
-#if FAIL_CASE_OPEN
 TEST_F(DenseUniTensorTest, put_block_out_of_idx) {
   constexpr cytnx_uint64 dim1 = 2, dim2 = 3;
   auto tens = zeros({dim1, dim2});
@@ -1507,7 +1502,6 @@ TEST_F(DenseUniTensorTest, put_block_out_of_idx) {
   auto ut = UniTensor({Bond(dim1), Bond(dim2)});
   EXPECT_THROW(ut.put_block(tens, 1), std::logic_error);
 }
-#endif
 
 /*=====test info=====
 describe:test put_block_
@@ -1592,7 +1586,6 @@ TEST_F(DenseUniTensorTest, put_block__rank_mismatch) {
 /*=====test info=====
 describe:test put_block_, out of index
 ====================*/
-#if FAIL_CASE_OPEN
 TEST_F(DenseUniTensorTest, put_block__out_of_idx) {
   constexpr cytnx_uint64 dim1 = 2, dim2 = 3;
   auto tens = zeros({dim1, dim2});
@@ -1603,7 +1596,6 @@ TEST_F(DenseUniTensorTest, put_block__out_of_idx) {
   auto ut = UniTensor({Bond(dim1), Bond(dim2)});
   EXPECT_THROW(ut.put_block_(tens, 1), std::logic_error);
 }
-#endif
 
 /*=====test info=====
 describe:test put_blocks, input uninitialized UniTensor