MySQL添加外键时报错：1215 Cannot add the foreign key constraint的解决方法

DROP TABLE IF EXISTS `product`; 
CREATE TABLE `product` ( 
 `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT, 
 `name` varchar(20) NOT NULL COMMENT 'product name', 
 `price` float(10,3) NOT NULL, 
 `description` varchar(20) DEFAULT NULL, 
 `count` int(11) NOT NULL DEFAULT '0', 
 `sid` int(11) NOT NULL, 
 PRIMARY KEY (`id`), 
 UNIQUE KEY `id_index` (`id`) USING HASH, 
 UNIQUE KEY `sid_index` (`sid`) USING HASH 
) ENGINE=InnoDB DEFAULT CHARSET=utf8; 

DROP TABLE IF EXISTS `sealer`; 
CREATE TABLE `sealer` ( 
 `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT, 
 `name` varchar(30) NOT NULL, 
 `city` varchar(255) DEFAULT NULL, 
 `created_time` datetime DEFAULT NULL, 
 `updated_time` datetime DEFAULT NULL, 
 `level` int(11) NOT NULL DEFAULT '0', 
 `description` varchar(40) DEFAULT NULL, 
 PRIMARY KEY (`id`), 
 UNIQUE KEY `id_index_1` (`id`) USING HASH 
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8; 

alter table `product' add CONSTRAINT `sid_ref` FOREIGN KEY (`sid`) REFERENCES `sealer` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION