[MM3-users] Different Mail Domain Cause Key Error

I have a client that requires his list address to be list@domain.com. But since he is using domain.com for an external website, he has to access his list via Postorius using list.domain.com. So here is what I did:

Django Admin Add Site: list.domain.com Add Mail Domain: domain.com (Site selected is list.domain.com)

Postorius Add Domain: Mailhost is domain.com. Web Host is list.domain.com (list.domain.com).

I have list.domain.com listed in the allowed hosts in settings_local.py. I even added domain.com there as well.

When I try to access Postorius using list.domain.com I get a "An error occurred while processing your request." message and the logs shows the dreaded:

'Internal Server Error: /mailman3/lists/

KeyError at /mailman3/lists/
'file'

Did I miss a step here?

Thanks, Brian