不能建立外键约束(can't create foreign key constraint)

CREATE TABLE share(
    `User_1` VARCHAR(50) NOT NULL,
    `User_2` VARCHAR(50) NOT NULL,
    PRIMARY KEY(`User_1`,`User_2`),
     CONSTRAINT `fk_1` FOREIGN KEY (`User_1`) REFERENCES user(`Dev_ID`)  ON DELETE RESTRICT ON UPDATE CASCADE ,
     CONSTRAINT `fk_2` FOREIGN KEY (`User_2`) REFERENCES user(`Dev_ID`)  ON DELETE RESTRICT ON UPDATE CASCADE 
);

我得到这个错误：

{外键约束的格式不正确}

我想让user1和user2之间的联系。

文章来源: can't create foreign key constraint