diff --git a/gruntfile.js b/gruntfile.js
index 036552dd4b..1841052a71 100644
--- a/gruntfile.js
+++ b/gruntfile.js
@@ -89,6 +89,7 @@ module.exports = function(grunt) {
           repos: [
             '.',
             '../platform',
+            '../WeakMap',
             '../ShadowDOM',
             '../HTMLImports',
             '../CustomElements',