diff --git a/actionpack/lib/action_dispatch/testing/assertions/response.rb b/actionpack/lib/action_dispatch/testing/assertions/response.rb
index 7381617dd7d631d86ab2016f44ab1a13d1472adb..fbee226a36e6995ae0a6d7f1eda8e74c7b9369fb 100644
--- a/actionpack/lib/action_dispatch/testing/assertions/response.rb
+++ b/actionpack/lib/action_dispatch/testing/assertions/response.rb
@@ -35,7 +35,7 @@ def assert_response(type, message = nil)
         elsif type.is_a?(Symbol) && @response.response_code == Rack::Utils::SYMBOL_TO_STATUS_CODE[type]
           assert_block("") { true } # to count the assertion
         else
-          flunk(build_message(message, "Expected response to be a <?>, but was <?>", type, @response.response_code))
+          flunk "Expected response to be a <#{type}>, but was <#{@response.response_code}>"
         end
       end