diff --git a/lib/pubsub/subscription.js b/lib/pubsub/subscription.js
index 82a2babcd1f..9e2abb50430 100644
--- a/lib/pubsub/subscription.js
+++ b/lib/pubsub/subscription.js
@@ -263,7 +263,7 @@ Subscription.prototype.ack = function(ids, callback) {
     ackIds: ids
   };
   var path = this.name + ':acknowledge';
-  this.makeReq_('POST', path, null, body, callback);
+  this.makeReq_('POST', path, null, body, callback || util.noop);
 };
 
 /**