diff --git a/test/serialization_test.rb b/test/serialization_test.rb
index 06c531eb..936ee113 100644
--- a/test/serialization_test.rb
+++ b/test/serialization_test.rb
@@ -227,7 +227,7 @@ class RenderJsonTest < ActionController::TestCase
 
   def test_render_json_nil_with_custom_serializer
     get :render_json_nil_with_custom_serializer
-    assert_equal 'null', @response.body
+    assert_equal "{\"dummy_custom\":null}", @response.body
   end
 
   def test_render_json