how to fix ??? Can't find FULLTEXT index matching the column list

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how to fix ??? Can't find FULLTEXT index matching the column list
pls help me
thanks

Database error in vBulletin 4.0.3:

Invalid SQL:

SELECT searchgroup.groupid, MATCH(searchgroup_text.title) AGAINST ('fsdfsdfsdf') AS score
FROM searchgroup AS searchgroup JOIN searchgroup_text AS searchgroup_text ON
(searchgroup.searchgroupid = searchgroup_text.searchgroupid)

WHERE MATCH(searchgroup_text.title) AGAINST ('fsdfsdfsdf') AND
searchgroup.contenttypeid = 2


HAVING score > 4
LIMIT 5;

MySQL Error : Can't find FULLTEXT index matching the column list
Error Number : 1191
Request Date : Thursday, June 24th 2010 @ 02:45:39 PM
Error Date : Thursday, June 24th 2010 @ 02:45:39 PM
Script : http://ss1.vn/newthread.php?do=postthread&f=56
Referrer : http://ss1.vn/newthread.php?do=newthread&f=56
IP Address : 113.22.48.60
Username : admin
Classname : vB_Database
MySQL Version : 5.0.45-community-nt