mysql 存储过程求助

delimiter //
create procedure fix_position()
begin
	declare t mediumint(8);
	declare b mediumint(8);
	declare c mediumint(8);
	DECLARE thread_id CURSOR FOR select tid from pre_forum_thread;
	open threadid;
		myloop: LOOP
		fetch thread_id into t;
		SELECT count(*) into b FROM pre_forum_post WHERE tid=t;
		set c= b - 1;
		UPDATE pre_forum_thread SET replies= c WHERE tid=t;
		UPDATE pre_forum_thread SET maxposition=b WHERE tid=t;
		END LOOP;
	CLOSE thread_id;
end;//
DELIMITER ;

discuz 的数据库，重新计算回复的次数然后更新到 pre_forum_thread 表， 只要执行就会报 Out of range value for column 'replies' at row 1 对 mysql 没啥研究，这还是根据网上资料现学的。。请问应该怎么修改？ 感谢