mysql - 无法创建外键约束

Question

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 
);

我收到此错误：

{外键约束格式不正确}

我想在 user1 和 user2 之间建立一个链接。