[Samba] Strange "username map" behaviour with "security=user"

[Vladimir Yumashev]

I use Samba 2.2.0 on small network.  I use "security = user" and "username map" to map 
some Win-users to unix users.
I have simple testing file with username mapping:
    root = vlad
It is supposed that when I connect to samba as user "Vlad" with vlad's password I get the root's rigths to shares.  Right?
But when I try to connect to samba as "vlad" it tries to authorize me as samba user "root" and tries to find user "root" in passdb.  Why?
I see the following lines in logfile:
--------
[2003/02/06 06:26:20, 1] smbd/password.c:pass_check_smb(545)
  Couldn't find user 'root' in passdb.
[2003/02/06 06:26:20, 1] smbd/password.c:pass_check_smb(545)
  Couldn't find user 'root' in passdb.
[2003/02/06 06:26:20, 1] smbd/reply.c:reply_sesssetup_and_X(1001)
  Rejecting user 'root': authentication failed
--------
Username mapping works as it is described in manual when I set "security = domain" and join samba to NT domain.  Then when I connect to samba as domain user "vlad" samba and domain controller authorize me as "vlad" against vlad's password.  Then samba maps me to root. So I have root's rigths to files in shares and logfile tells me:
-----
[2003/02/06 06:55:40, 1] smbd/service.c:make_connection(636)
  vladimir (192.168.100.10) connect to service tmp as user root (uid=0, gid=0) (pid 31605)
-----

Thanks,
Vladimir