diff --git a/lib/socket.js b/lib/socket.js
index d9807f6dd2..d8658c9266 100644
--- a/lib/socket.js
+++ b/lib/socket.js
@@ -1,4 +1,3 @@
-
 /*!
  * socket.io-node
  * Copyright(c) 2011 LearnBoost <dev@learnboost.com>
@@ -358,7 +357,7 @@ Socket.prototype.emit = function (ev) {
 
   if ('function' == typeof lastArg) {
     packet.id = ++this.ackPackets;
-    packet.ack = lastArg.length ? 'data' : true;
+    packet.ack = 'data';
     this.acks[packet.id] = lastArg;
     args = args.slice(0, args.length - 1);
   }