diff --git a/debugger_protocol/schema/upstream.py b/debugger_protocol/schema/upstream.py
index 48a6fa2d..b46a057d 100644
--- a/debugger_protocol/schema/upstream.py
+++ b/debugger_protocol/schema/upstream.py
@@ -28,7 +28,7 @@ def read(url, *, _open_url=open_url):
     try:
         infile = _open_url(url)
     except (FileNotFoundError, urllib.error.HTTPError) as exc:
-        # XXX Ensure it's a 404 error?
+        # TODO: Ensure it's a 404 error?
         raise SchemaFileError('schema file at {!r} not found'.format(url))
     with infile:
         upstream = download(url, infile, outfile, _open=_open_url)