NFS root. Caanot log in ...

So this is odd. Using TFTP boot and an export NFS file system, board boots and initializes correctly. I then get the usual prompt:

Debian GNU/Linux 8 arm ttyO0

default username:password is [debian:temppwd]

arm login: debian


But the login fails. Is there anything else that needs to be configured in the exported file system? The FS in question is the standard Debian Jessie image.