Fixing Rails::Server#app under Rack::URLMap

railties/lib/rails/application.rb

@@ -138,6 +138,10 @@ module Rails
       @config ||= Application::Configuration.new(find_root_with_flag("config.ru", Dir.pwd))
     end
 
+    def to_app
+      self
+    end
+
   protected
 
     alias :build_middleware_stack :app

railties/lib/rails/commands/server.rb

@@ -43,7 +43,7 @@ module Rails
     end
 
     def app
-      @app ||= super.instance
+      @app ||= super.respond_to?(:to_app) ? super.to_app : super
     end
 
     def opt_parser