[Samba] error when adding a new user

[Guido Lorenzutti]

Hi people, i having the following problem:

useradd -s /bin/false -g users test
smbpasswd -a test
New SMB password:
Retype new SMB password:
tdb_update_sam: Failing to store a SAM_ACCOUNT for [test] without a 
primary group RID
Failed to add entry for user test.
Failed to modify password entry for user test

The "users" group exists,

grep users /etc/group
users:x:100:

Any ideas? I fix this adding a new group with the name of the username 
and adding the user to this group. Then, it works de smbpasswd -a.
Why this could happend?