diff --git a/actionview/test/template/form_helper_test.rb b/actionview/test/template/form_helper_test.rb
index e4fd7979244a24e9fb44dce903c45a45e91b846c..dccf6a209046ed7a6a177de79986e24be7811df4 100644
--- a/actionview/test/template/form_helper_test.rb
+++ b/actionview/test/template/form_helper_test.rb
@@ -910,7 +910,11 @@ def test_inputs_use_before_type_cast_to_retain_information_from_validations_like
   end
 
   def test_inputs_dont_use_before_type_cast_when_value_did_not_come_from_user
-    def @post.id_came_from_user?; false; end
+    class << @post
+      undef id_came_from_user?
+      def id_came_from_user?; false; end
+    end
+
     assert_dom_equal(
       %{<textarea id="post_id" name="post[id]">\n0</textarea>},
       text_area("post", "id")