diff -r b521b3a79afd -r e21f274cccea mercurial/hgweb/protocol.py
--- a/mercurial/hgweb/protocol.py	Thu Oct 05 14:18:55 2017 -0700
+++ b/mercurial/hgweb/protocol.py	Thu Oct 05 14:12:51 2017 -0400
@@ -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)