为什么MYSQL外键references老是报错？

CREATE TABLE Users(
uid int(11) unsigned NOT NULL auto_increment,
name varchar(11) NOT NULL default '',
password int(6) NOT NULL default 0,
login_id varchar(11) NOT NULL default '',

PRIMARY KEY(uid)
)

CREATE TABLE Students(
uid int(11) unsigned NOT NULL,
e_mail varchar(80) NOT NULL default '',
phone varchar(11) NOT NULL default '',

PRIMARY KEY(uid),
FOREIGH KEY(uid) REFERENCES Users(uid)
on delete cascade on update cascade
)
老是报错，

取消on delete cascade on update cascade约束也没用，

请问是什么原因？

相关了解……