diff --git a/testing/web-platform/tests/tools/manifest/manifest.py b/testing/web-platform/tests/tools/manifest/manifest.py
index b510006b757e..8bbabc38a445 100644
--- a/testing/web-platform/tests/tools/manifest/manifest.py
+++ b/testing/web-platform/tests/tools/manifest/manifest.py
@@ -51,6 +51,15 @@ class Manifest(object):
             for test in type_tests.get(path, set()):
                 yield test
 
+    def iterdir(self, dir_name):
+        if not dir_name.endswith(os.path.sep):
+            dir_name = dir_name + os.path.sep
+        for type_tests in self._data.values():
+            for path, tests in type_tests.iteritems():
+                if path.startswith(dir_name):
+                    for test in tests:
+                        yield test
+
     @property
     def reftest_nodes_by_url(self):
         if self._reftest_nodes_by_url is None:
diff --git a/testing/web-platform/tests/tools/wptrunner/wptrunner/manifestinclude.py b/testing/web-platform/tests/tools/wptrunner/wptrunner/manifestinclude.py
index f7229d504f13..18d034e34ae3 100644
--- a/testing/web-platform/tests/tools/wptrunner/wptrunner/manifestinclude.py
+++ b/testing/web-platform/tests/tools/wptrunner/wptrunner/manifestinclude.py
@@ -93,7 +93,8 @@ class IncludeManifest(ManifestItem):
                 for manifest, data in test_manifests.iteritems():
                     found = False
                     rel_path = os.path.relpath(path, data["tests_path"])
-                    for test in manifest.iterpath(rel_path):
+                    iterator = manifest.iterpath if os.path.isfile(path) else manifest.iterdir
+                    for test in iterator(rel_path):
                         if not hasattr(test, "url"):
                             continue
                         url = test.url