diff --git a/mercurial/hgweb/protocol.py b/mercurial/hgweb/protocol.py
--- a/mercurial/hgweb/protocol.py
+++ b/mercurial/hgweb/protocol.py
@@ -15,6 +15,7 @@
 )
 
 from .. import (
+    error,
     util,
     wireproto,
 )
@@ -199,3 +200,4 @@
         rsp = rsp.message
         req.respond(HTTP_OK, HGERRTYPE, body=rsp)
         return []
+    raise error.ProgrammingError('hgweb.protocol internal failure', rsp)