diff --git a/lib/socket.js b/lib/socket.js
index 7e13a6125..64e3f49bb 100644
--- a/lib/socket.js
+++ b/lib/socket.js
@@ -244,6 +244,7 @@ Socket.prototype.ack = function(id){
   return function(){
     // prevent double callbacks
     if (sent) return;
+    sent = true;
     var args = toArray(arguments);
     debug('sending ack %j', args);
     self.packet({