--- a/testing/marionette/marionette-simpletest.js
+++ b/testing/marionette/marionette-simpletest.js
@@ -170,17 +170,17 @@ Marionette.prototype = {
   waitFor: function test_waitFor(callback, test, timeout) {
       this.heartbeatCallback();
       if (test()) {
         callback();
         return;
       }
       var now = new Date();
       var deadline = (timeout instanceof Date) ? timeout :
-                     new Date(now.valueOf + (typeof(timeout) == "undefined" ? this.timeout : timeout))
+                     new Date(now.valueOf() + (typeof(timeout) == "undefined" ? this.timeout : timeout))
       if (deadline <= now) {
         dump("waitFor timeout: " + test.toString() + "\n");
         // the script will timeout here, so no need to raise a separate
         // timeout exception
         return;
       }
       this.window.setTimeout(this.waitFor.bind(this), 100, callback, test, deadline);
   },