diff --git a/actionpack/lib/action_dispatch/test/mock.rb b/actionpack/lib/action_dispatch/test/mock.rb
index 4042f9fbc8e6f..68e5b108b46a3 100644
--- a/actionpack/lib/action_dispatch/test/mock.rb
+++ b/actionpack/lib/action_dispatch/test/mock.rb
@@ -42,6 +42,7 @@ def env_for(path, opts)
               opts[:input] = params
             end
           else
+            params.stringify_keys!
             params.update(::Rack::Utils.parse_query(uri.query))
             uri.query = requestify(params)
           end