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 bb2_threads AS a,bb2_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: 5.7.19
php version: 5.6.31
Date: 24.09.2017 @ 10:35
Script: /forum/statistik.php?sid=bb95c094f74b04f44c54ccb82f64c539
Referer: