SQL-DATABASE ERROR

Database error in WoltLab Burning Board (2.1.3): Invalid SQL: SELECT a.replycount AS replies,a.boardid AS boardparentid,a.threadid AS threadid,a.topic AS threadname FROM bb3_threads AS a,bb3_boards AS b WHERE a.boardid = b.boardid ORDER BY a.replies DESC
mysql error: Unknown column 'a.replies' in 'order clause'
mysql error number: 1054
mysql version: 5.0.51a-3ubuntu5.7-log
php version: 5.2.12-nmm2
Date: 09.09.2010 @ 12:18
Script: /cp3/statistik.phpx?sid=$session[hash]
Referer: