MySQL error

[Smoothie]

here's the error:
Invalid SQL:
SELECT signature
FROM user
WHERE userid=
LIMIT 1

mysql error: You have an error in your SQL syntax near 'LIMIT 1
' at line 5

mysql error number: 1064
Here's the code:

PHP Code:

$userinfo=$DB_site->query_first("
    SELECT signature
    FROM user
       WHERE userid=$bbuserid
       LIMIT 1
       "); 


It's only happening to one user.