security=server / username map / valid users NOT working

[Tom]

I have a share set up in Samba so only 3 users can get to it.

[test-share]
  path=/home/tom/test-dir
  valid users = user1, user2, user3
  read only = yes
  printable = no


user1 has an account on the UNIX box itself and can connect no problem.

user2 and user3 have accounts on the NT domain.
Security is set to 'server' and password server set to the PDC and
BDC's.

A username map is set up as follows:

nobody = user2 user3

So it should map to 'nobody' when they connect.
When they do try to connect, they get prompted for a username and
password (which of course doesn't work.)

Is there a problem with mapping users to 'nobody' in a username map,
and using "valid users"?

Thanks in advance,
Tom

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