diff --git a/debugger_protocol/schema/_util.py b/debugger_protocol/schema/_util.py
index dbb70781..79d08a77 100644
--- a/debugger_protocol/schema/_util.py
+++ b/debugger_protocol/schema/_util.py
@@ -14,7 +14,7 @@ def get_revision(url, *, _open_url=open_url):
     if url.startswith('https://github.com/'):
         return github_get_revision(url, _open_url=_open_url)
     else:
-        raise NotImplementedError
+        return '<unknown>'
 
 
 def get_checksum(data):
diff --git a/tests/debugger_protocol/schema/test_util.py b/tests/debugger_protocol/schema/test_util.py
index e048105c..f636a36c 100644
--- a/tests/debugger_protocol/schema/test_util.py
+++ b/tests/debugger_protocol/schema/test_util.py
@@ -14,6 +14,12 @@ class GetRevisionTests(unittest.TestCase):
 
         self.assertEqual(revision, 'fc2395ca3564fb2afded8d90ddbe38dad1bf86f1')
 
+    def test_unrecognized_url(self):
+        revision = get_revision('https://localhost/schema.json',
+                                _open_url=lambda _: io.BytesIO())
+
+        self.assertEqual(revision, '<unknown>')
+
 
 class GetChecksumTests(unittest.TestCase):