diff --git a/activitypub.py b/activitypub.py
index 425931f..e62ddd9 100644
--- a/activitypub.py
+++ b/activitypub.py
@@ -715,7 +715,8 @@ def postprocess_as2_actor(actor, wrap=True):
 def actor(handle_or_id):
     """Serves a user's AS2 actor from the datastore."""
     cls = Protocol.for_request(fed=PROTOCOLS['web'])
-    assert cls
+    if not cls:
+        error(f"Couldn't determine protocol", status=404)
 
     if cls.owns_id(handle_or_id) is False:
         if cls.owns_handle(handle_or_id) is False:
diff --git a/tests/test_activitypub.py b/tests/test_activitypub.py
index c80636e..5eec280 100644
--- a/tests/test_activitypub.py
+++ b/tests/test_activitypub.py
@@ -417,6 +417,11 @@ class ActivityPubTest(TestCase):
         got = self.client.get('/ap/fake/fake:handle:nope')
         self.assertEqual(404, got.status_code)
 
+    def test_actor_no_matching_protocol(self, *_):
+        resp = self.client.get('/foo.json',
+                               base_url='https://bridgy-federated.appspot.com/')
+        self.assertEqual(404, resp.status_code)
+
     def test_individual_inbox_no_user(self, mock_head, mock_get, mock_post):
         self.user.key.delete()
 
@@ -437,6 +442,10 @@ class ActivityPubTest(TestCase):
         resp = self.post('/ap/sharedInbox', json=note)
         self.assertEqual(400, resp.status_code)
 
+    def test_inbox_no_matching_protocol(self, *_):
+        resp = self.post('/foo.json/inbox', json=NOTE)
+        self.assertEqual(400, resp.status_code)
+
     def test_inbox_reply_object(self, mock_head, mock_get, mock_post):
         self._test_inbox_reply(REPLY_OBJECT, mock_head, mock_get, mock_post)