imap: invalid header in response while in http auth state

[Grant]

I'm using imapproxy and trying to switch to nginx.  courier is
listening on port 143.

mail {
        auth_http localhost:143;
        proxy on;
        server {
                listen 144;
                protocol imap;
        }
}

I get:

auth http server 127.0.0.1:143 sent invalid header in response while
in http auth state, client: 127.0.0.1, server: 0.0.0.0:144

Does anyone know what's wrong?

- Grant