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: 5.7.20
php version: 5.6.32
Date: 20.11.2017 @ 23:42
Script: /forum/statistik.php?sid=52626e5c2fd037e74351752ef0c43dac
Referer: