diff -r d4c760c997cd -r 7574c8173d5e mercurial/wireprotoserver.py
--- a/mercurial/wireprotoserver.py	Tue Mar 06 02:43:17 2018 -0600
+++ b/mercurial/wireprotoserver.py	Tue Mar 06 15:02:53 2018 -0800
@@ -235,14 +235,14 @@
         for chunk in gen:
             yield chunk
 
-    rsp = wireproto.dispatch(repo, proto, cmd)
-
     if not wireproto.commands.commandavailable(cmd, proto):
         req.respond(HTTP_OK, HGERRTYPE,
                     body=_('requested wire protocol command is not available '
                            'over HTTP'))
         return []
 
+    rsp = wireproto.dispatch(repo, proto, cmd)
+
     if isinstance(rsp, bytes):
         req.respond(HTTP_OK, HGTYPE, body=rsp)
         return []