using subselects

Hello everyone -

I'm moving from MySQL to Postgres and I am in a situation where I am
trying to use subselects properly.

I have a table of projects, users, and user_projects. The table
user_projects creates a many-to-many relationship between users and
projects.

I'm creating a select list on a web form for adding a new user to a
project. I want to select users from the user table, who aren't
already listed in the join table under that project.

Here's my select:
SELECT * FROM users
WHERE user_id <> $current_user_id
AND user_id <> ( SELECT user_id FROM user_projects WHERE project_id =
$project_id )

This query returns no rows, even on projects that have no records in
the user_projects table!

I am certain that I am not the $current_user_id. If I run this query:
SELECT * FROM users
WHERE user_id <> $current_user_id

I get all the user records besides myself.

What am I doing wrong?