[Samba] Failed to initialise SAM_ACCOUNT for user

[Samba]

I'm trying to create samba user accounts for users that don't have 
accounts on the machine.  I get the following error when using 
'smbpasswd -a <username>'. 

"Failed to initialise SAM_ACCOUNT for user <username>.  Does this user 
exist in the UNIX password database?"

Am I doing something wrong?  Or can you not have samba accounts if they 
don't have user accounts.

I'm using samba 3.0.7 on FreeBSD 5.3.

Thanks in advance.