database error when posting message
 

I recently get this error:

Database error in vBulletin 3.8.2:
Invalid SQL:
SELECT thread.threadid, MATCH(thread.title) AGAINST ('test subject') AS score
FROM vb_thread AS thread

WHERE MATCH(thread.title) AGAINST ('test subject')
AND thread.open 10


LIMIT 5;
MySQL Error : Can't find FULLTEXT index matching the column list
Error Number : 1191
Request Date : Saturday, January 2nd 2010 @ 03:04:08 AM
Error Date : Saturday, January 2nd 2010 @ 03:04:08 AM
Script : /messageboards/newthread.php?do=postthread&f=534
Referrer : /messageboards/newthread.php?do=newthread&f=534
IP Address :
Username : admin
Classname : vB_Database
MySQL Version : 5.0.51a-community

Any ideas on how to fix it, and what caused the new error?

I tried
ALTER TABLE vb_post ADD FULLTEXT KEY `title` (`title`,`pagetext`)
as per another post.

Thanks.