NTLM challenge response not accepted

[aa403]

Hi - I'm running a fastapi server which needs to validate ntlm auth from a client.

using spnego.server (set up like the unittests show), the initial negotiation works fine, but when the client (in this case google chrome) returns the challenge response (the second hash), the server.step call throws the following error:

"Input message was not a NTLM Negotiate message"

any ideas?