diff --git a/tools/bundler.js b/tools/bundler.js
index 11d6df16c5..56eb8bd3f7 100644
--- a/tools/bundler.js
+++ b/tools/bundler.js
@@ -1291,7 +1291,7 @@ _.extend(JsImage.prototype, {
       };
 
       if (!assets || !_.has(assets, assetPath)) {
-        _.callback(new Error("Unknown asset: " + assetPath));
+        _callback(new Error("Unknown asset: " + assetPath));
       } else {
         var buffer = assets[assetPath];
         var result = encoding ? buffer.toString(encoding) : buffer;