--- a/testing/web-platform/tests/tools/wptrunner/wptrunner/update/update.py
+++ b/testing/web-platform/tests/tools/wptrunner/wptrunner/update/update.py
@@ -25,17 +25,17 @@ class LoadConfig(Step):
         state.sync = {"remote_url": state.kwargs["remote_url"],
                       "branch": state.kwargs["branch"],
                       "path": state.kwargs["sync_path"]}
 
         state.paths = state.kwargs["test_paths"]
         state.tests_path = state.paths["/"]["tests_path"]
         state.metadata_path = state.paths["/"]["metadata_path"]
 
-        assert state.tests_path.startswith("/")
+        assert os.path.isabs(state.tests_path)
 
 
 class LoadTrees(Step):
     """Step for creating a Tree for the local copy and a GitTree for the
     upstream sync."""
 
     provides = ["local_tree", "sync_tree"]