diff --git a/railties/lib/rails/engine.rb b/railties/lib/rails/engine.rb
index 457a50e649..a9d98a9d92 100644
--- a/railties/lib/rails/engine.rb
+++ b/railties/lib/rails/engine.rb
@@ -508,7 +508,8 @@ module Rails
     def app
       @app || @app_build_lock.synchronize {
         @app ||= begin
-          config.middleware = build_middleware.merge_into(default_middleware_stack)
+          stack = default_middleware_stack
+          config.middleware = build_middleware.merge_into(stack)
           config.middleware.build(endpoint)
         end
       }