diff --git a/testing/mochitest/chunkifyTests.js b/testing/mochitest/chunkifyTests.js
index 0b1f23f6738..7a5b89594b1 100644
--- a/testing/mochitest/chunkifyTests.js
+++ b/testing/mochitest/chunkifyTests.js
@@ -15,10 +15,10 @@ function chunkifyTests(tests, totalChunks, thisChunk, chunkByDir, logger) {
     var tests_by_dir = {};
     var test_dirs = []
     for (var i = 0; i < tests.length; ++i) {
-      if ('test' in tests[i]) {
+      if ((tests[i] instanceof Object) && ('test' in tests[i])) {
         var test_path = tests[i]['test']['url'];
       } else {
-        var test_path = tests[i]['url'];
+        var test_path = tests[i];
       }
       if (test_path[0] == '/') {
         test_path = test_path.substr(1);