diff --git a/lib/es6-promise/polyfill.js b/lib/es6-promise/polyfill.js
index 30db73c..f4ffd8d 100644
--- a/lib/es6-promise/polyfill.js
+++ b/lib/es6-promise/polyfill.js
@@ -27,7 +27,9 @@ export default function polyfill() {
     }
 
     if (promiseToString === '[object Promise]' && !P.cast){
-      return;
+      if (typeof P.prototype.finally !== "undefined"){
+        return;
+      }
     }
   }