CMS Database error at Install / Upgrade CMS when upgrading from 3.8.4 to 4.0.5
 

I got this following error message at http://[path-to-forum]/install/finalupgrade.php?step=6:
--------------------------------------------------------

Database error in vBulletin 4.0.5:
Invalid SQL:
ALTER TABLE cms_widget DROP INDEX title, ADD INDEX title(title);
MySQL Error : Table '[database-name].cms_widget' doesn't exist
Error Number : 1146
Request Date : [hidden]
Error Date : [hidden]
Script : http://[path-to-forum]/install/finalupgrade.php?step=6
Referrer : http://[path-to-forum]/install/finalupgrade.php?step=5
IP Address : [hidden]
Username :
Classname : vB_Database
MySQL Version : 5.0.91-community
--------------------------------------------------------
May anyone help me to get through this problem? I tried to post in the Tracker but there is no response for days.