SQL查询会产生SQL异常，但在DBMS中运行它是没有错误的

我正在使用PDO执行以下SQL查询，该查询应该与5 users和我预先检索到的ID的单个mod匹配：

$sql =  'INSERT INTO modusergrade (`ModId`, `UserId`)
             SELECT :modId AS ModId, u.Id AS UserId
             FROM users AS u
             LEFT OUTER JOIN modusergrade AS g ON g.UserId = u.Id
             WHERE u.Id NOT IN
                              (SELECT UserId
                               FROM moduserconflicts
                               WHERE ModId = :modId)
             GROUP BY u.Id
             ORDER BY COUNT(g.ModId), rand()
             LIMIT :maxUsers';
$query = $db->prepare($sql);
$query->bindParam(':modId', $id, PDO::PARAM_INT);
$query->bindParam(':maxUsers', $maxUsers, PDO::PARAM_INT);
$query->execute();

然而，执行此操作会产生以下错误：

SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near "5" at line 11

第11行对应于LIMIT，所以我假设rand()存在某种问题。我应该通过其他方式来获得"随机订单"效果吗？