diff --git a/toolkit/components/workerloader/require.js b/toolkit/components/workerloader/require.js
index 56a429da144..c3fd3b08524 100644
--- a/toolkit/components/workerloader/require.js
+++ b/toolkit/components/workerloader/require.js
@@ -109,7 +109,7 @@
       substitute: function(source) {
         let map = this._map;
         return source.replace(this.regexp, function(url) {
-          return map.get(url);
+          return map.get(url) || url;
         }, "g");
       }
     };