Question regarding 'not in' and subselects

Hi.  I'm using PostgreSQL 7.4.2 under Fedora Core 1.

I have two tables with a single varchar(32) column in each.  I'm trying
to find all the rows from one table that don't exist in the other
table.  The query that I am using is:

select u.user_name from users u where u.user_name not in (select
user_name from iasusers);

(Actually, I'm doing something a bit more complex but this illustrates
the problem.)

It always seems to return 0 rows.  As a test, I inserted a row into
users that I knew wasn't in iasuses but it didn't make a difference.

If I remove the 'not', the query returns the rows that exist in both
tables.

If I replace the subselect with a list, it seems to work the way that
I'd expect, i.e. "not in ('vic', 'joe')" it shows all the rows from
users except for vic and joe.

Am I doing something wrong or is this a bug?

Thanks,
-Vic