跨数据库外键，1215错误

我试图创建db1表read_x_tag_0列外键db2列下面是我的代码，不确定我做对了吗？它显示1215错误，找不到原因？

数据库“读取”
表“ read_x_tag_0”

CREATE TABLE `read_x_tag_0` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `read_id` int(11) unsigned NOT NULL,
  `tag_0_id` int(11) unsigned NOT NULL,
  `tag_0_sequence` int(11) unsigned NOT NULL,
  PRIMARY KEY (`id`),
  KEY `read_id` (`read_id`),
  KEY `tag_0_id` (`tag_0_id`),
  CONSTRAINT `read_x_tag_0_ibfk_1` FOREIGN KEY (`read_id`) REFERENCES `read` (`id`) ON DELETE CASCADE,
  CONSTRAINT `read_x_tag_0_ibfk_2` FOREIGN KEY (`tag_0_id`) REFERENCES `tag_0`.`tag_0` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=76 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

数据库“读取”
表“读取”

CREATE TABLE `read` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `subject` varchar(255) COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
  `intro` text COLLATE utf8_unicode_ci,
  `content` text COLLATE utf8_unicode_ci,
  `sequence` int(11) unsigned NOT NULL,
  `visible` int(11) unsigned NOT NULL COMMENT '1:visible,0:invisible',
  `create_at` datetime NOT NULL,
  `create_by` varchar(255) COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
  `update_at` datetime NOT NULL,
  `update_by` varchar(255) COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

数据库“标签_0”
表“标签_0”

CREATE TABLE `tag_0` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `subject` varchar(255) COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
  `visible` int(11) unsigned NOT NULL COMMENT '1:visible,0:invisible',
  `sequence` int(11) unsigned NOT NULL,
  `create_at` datetime NOT NULL,
  `create_by` varchar(255) COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
  `update_at` datetime NOT NULL,
  `update_by` varchar(255) COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
  PRIMARY KEY (`id`),
  UNIQUE KEY `subject` (`subject`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;