create a bunch of bogus users

[monotreme]

I hacked about 500 users from a proprietery system into vbulletin by simply creating records for them in the user table and stuffing them with reasonable values when there was no correspond in the old table. But there's something i obviously failed to set up because the "legit" users can't send pms to them. I get a database error thus

Invalid SQL:
INSERT INTO vb_pm (pmtextid, userid) VALUES (532, );

the missing value is supposed to be their userid, which does exist in the user table and is correct. but it needs to be somewhere else. Reading vb code is rather excrutiating. What little relation table did I overlook please?