mysql外键错误无法修复

我是my sql新手,从mysql数据库中编写sql脚本,我的大多数表都能工作,但有些表不能工作,mysql错误是尽可能不可描述的。这可能与多个外键有关。

 create table Users (
    id int not null auto_increment,
    type enum('t1', 't2', 't3') not null,
    name char(30) not null,
    phone char(15),
    email char(30),
    username char(30),
    password char(30),
    created timestamp,
    primary key (id)
);

create table Groups (
    id int not null auto_increment,
    user int not null,
    name char(30) not null,
    phone char(15),
    email char(30),
    created timestamp,
    primary key (id),
    foreign key (user) references Users(id)
);

create table Group_Members (
    id int not null auto_increment,
    group int not null,
    user int not null,
    created timestamp,
    primary key (id),
    foreign key (group) references Groups(id),
    foreign key (user) references Users(id)
);