select * from users where user_id NOT in (select ban_userid from banlist)

Hello,

I have this strange problem that the following statement works:

phpbb=> select user_id, username from phpbb_users
phpbb-> where user_id in (select ban_userid from phpbb_banlist);
user_id | username
---------+----------
3 | La-Li
(1 row)

But the negative one returns nothing:

phpbb=> select user_id, username from phpbb_users
phpbb-> where user_id not in (select ban_userid from phpbb_banlist);
user_id | username
---------+----------
(0 rows)

Eventhough there are 3 other users in the phpbb_users table:

phpbb=> select user_id, username from phpbb_users;
user_id | username
---------+-----------
-1 | Anonymous
3 | La-Li
4 | Vasja
2 | Alex
(4 rows)

And there is only one user (La-Li, id=3) in the phpbb_banlist:

phpbb=> select * from phpbb_banlist;
ban_id | ban_userid | ban_ip | ban_email
--------+------------+--------+-------------
1 | 3 | |
4 | | | *(at)gmail(dot)com
(2 rows)

Thank you
Alex