Upgrading Drupal

Can't seem to solve this:

[code]
user error: Unknown column 'c.thread' in 'field list'
query: SELECT c.cid as cid, c.pid, c.nid, c.subject, c.comment, c.timestamp, u.uid, u.name, u.data, c.score, c.users, c.thread FROM comments c INNER JOIN users u ON c.uid = u.uid WHERE c.nid = '15' AND c.status = 0 GROUP BY c.cid, c.pid, c.nid, c.subject, c.comment, c.timestamp, u.uid, u.name, u.data, c.score, c.users, c.thread ORDER BY c.thread DESC LIMIT 0, 50 in (...)/public_html/includes/database.mysql.inc on line 90.
[/code]

any suggestions?