diff --git a/src/React/Espresso/Stack.php b/src/React/Espresso/Stack.php
index 93ac52d..c7a1663 100644
--- a/src/React/Espresso/Stack.php
+++ b/src/React/Espresso/Stack.php
@@ -22,7 +22,7 @@ public function __construct($app)
             return new HttpServer($stack['socket']);
         });
 
-        $isFactory = is_object($app) && method_exists($app, '__invoke');
+        $isFactory = is_callable($app);
         $this['app'] = $isFactory ? $this->protectService($app) : $app;
     }