向该表添加多个外键约束时遇到问题。我听到一个错误说
外键约束格式不正确
引用的两个id都是各自表中的主键。我做错什么了？

CREATE TABLE `works_on` (
    `eid` int(11) DEFAULT NULL,
    `pid` int(11) DEFAULT NULL,
    `start_date` date NOT NULL,
    PRIMARY KEY (`eid`, `pid`),
    CONSTRAINT `works_on_ibfk_1` FOREIGN KEY (`eid`)
    REFERENCES `employee` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
    CONSTRAINT `works_on_ibfk_2` FOREIGN KEY (`pid`) 
    REFERENCES `project` (`id`) ON DELETE SET NULL ON UPDATE CASCADE
) ENGINE=InnoDB