新安装2.5版时怎么会报错?

执行查询错误: CREATE TABLE mxb_badword ( id smallint(5) unsigned NOT NULL auto_increment, badbefore varchar(250) NOT NULL default '', badafter varchar(250) NOT NULL default '', type tinyint(1) NOT NULL default '0', PRIMARY KEY (id), KEY badbefore (badbefore, type) )

MySQL 返回结果: Specified key was too long. Max key length is 500

是怎么回事？

表里有varchar(250) 就会报错！怎么解决啊？