diff --git a/core/extensions.js b/core/extensions.js
index 1ce582cc1..c32e1d60f 100644
--- a/core/extensions.js
+++ b/core/extensions.js
@@ -74,6 +74,9 @@ Blockly.Extensions.register = function(name, initFn) {
  *     registered.
  */
 Blockly.Extensions.registerMixin = function(name, mixinObj) {
+  if(!goog.isObject(mixinObj)){
+    throw new Error('Error: Mixin "' + name + '" must be a object');
+  }
   Blockly.Extensions.register(name, function() {
     this.mixin(mixinObj);
   });