Check if run callback is specific before setting it as the promise ha…

…ndler

Fix #1086

lib/index.js

@@ -486,7 +486,11 @@ class Generator extends EventEmitter {
       _.invokeMap(this._composedWith, 'run');
     });
 
-    promise.then(cb, cb);
+    // Maintain backward compatibility with the callback function
+    if (_.isFunction(cb)) {
+      promise.then(cb, cb);
+    }
+
     return promise;
   }