diff --git a/ext/zstdruby/extconf.rb b/ext/zstdruby/extconf.rb
index f963367..ef976a6 100644
--- a/ext/zstdruby/extconf.rb
+++ b/ext/zstdruby/extconf.rb
@@ -26,7 +26,7 @@ def ext_export_filename
   name
 end
 
-$CFLAGS = '-I. -O3 -std=c99 -DZSTD_STATIC_LINKING_ONLY -DZSTD_MULTITHREAD -pthread -DDEBUGLEVEL=0 -fvisibility=hidden -DZSTDLIB_VISIBLE=\'__attribute__((visibility("hidden")))\' -DZSTDLIB_HIDDEN=\'__attribute__((visibility("hidden")))\''
+$CFLAGS += ' -I. -O3 -std=c99 -DZSTD_STATIC_LINKING_ONLY -DZSTD_MULTITHREAD -pthread -DDEBUGLEVEL=0 -fvisibility=hidden -DZSTDLIB_VISIBLE=\'__attribute__((visibility("hidden")))\' -DZSTDLIB_HIDDEN=\'__attribute__((visibility("hidden")))\''
 $CPPFLAGS += " -fdeclspec" if CONFIG['CXX'] =~ /clang/
 
 # macOS specific: Use exported_symbols_list to control symbol visibility