diff --git a/lib/ace/lib/oop.js b/lib/ace/lib/oop.js
index f9ba6fe8..f3dbb1d9 100644
--- a/lib/ace/lib/oop.js
+++ b/lib/ace/lib/oop.js
@@ -45,6 +45,7 @@ exports.mixin = function(obj, mixin) {
     for (var key in mixin) {
         obj[key] = mixin[key];
     }
+    return obj;
 };
 
 exports.implement = function(proto, mixin) {