diff --git a/lib/_http_outgoing.js b/lib/_http_outgoing.js
index dde832ee072..283a5ab824b 100644
--- a/lib/_http_outgoing.js
+++ b/lib/_http_outgoing.js
@@ -89,8 +89,12 @@ exports.OutgoingMessage = OutgoingMessage;
 
 
 OutgoingMessage.prototype.setTimeout = function(msecs, callback) {
-  if (callback)
+
+  if (callback) {
+    if (typeof callback !== 'function')
+      throw new TypeError('callback must be a function');
     this.on('timeout', callback);
+  }
 
   if (!this.socket) {
     this.once('socket', function(socket) {