EventEmitter#once only takes instanceof function

lib/events.js

@@ -137,6 +137,10 @@ EventEmitter.prototype.addListener = function(type, listener) {
 EventEmitter.prototype.on = EventEmitter.prototype.addListener;
 
 EventEmitter.prototype.once = function(type, listener) {
+  if ('function' !== typeof listener) {
+    throw new Error('.once only takes instances of Function');
+  }
+
   var self = this;
   self.on(type, function g() {
     self.removeListener(type, g);