--- a/dom/animation/test/testcommon.js
+++ b/dom/animation/test/testcommon.js
@@ -269,17 +269,19 @@ if (opener) {
   for (var funcName of ["async_test", "assert_not_equals", "assert_equals",
                         "assert_approx_equals", "assert_less_than",
                         "assert_less_than_equal", "assert_greater_than",
                         "assert_between_inclusive",
                         "assert_true", "assert_false",
                         "assert_class_string", "assert_throws",
                         "assert_unreached", "assert_regexp_match",
                         "promise_test", "test"]) {
-    window[funcName] = opener[funcName].bind(opener);
+    if (opener[funcName]) {
+      window[funcName] = opener[funcName].bind(opener);
+    }
   }
 
   window.EventWatcher = opener.EventWatcher;
 
   function done() {
     opener.add_completion_callback(function() {
       self.close();
     });