Не удается добавить foreign key

Пытаюсь добавить FK:

set foreign_key_checks = 0;
ALTER TABLE `science_redirects` 
ADD CONSTRAINT `fk-contest_id` 
FOREIGN KEY (`contest_id`) 
REFERENCES `contests` (`id`) 
ON DELETE CASCADE;
set foreign_key_checks = 1;

и мне пишет ошибку: [Err] 1823 - Failed to add the foreign key constraint 'fk-contest_id' to system tables. И так еще на куче ключей которые я пытаюсь добавить. Подскажите в чем дело?

DDL:

CREATE TABLE `science_redirects` (
  `id` int(11) NOT NULL,
  `old_id` int(11) NOT NULL,
  `contest_id` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
CREATE TABLE `contests` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `number` smallint(6) DEFAULT NULL,
  `is_active` tinyint(1) NOT NULL DEFAULT '0',
  `is_completed` tinyint(1) NOT NULL DEFAULT '0',
  `show_certificates` tinyint(1) NOT NULL DEFAULT '0',
  `article_price` int(11) unsigned NOT NULL DEFAULT '0',
  `adviser_fullname` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `adviser_phone` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `main_text` longtext COLLATE utf8_unicode_ci,
  `about_text` longtext COLLATE utf8_unicode_ci,
  PRIMARY KEY (`id`),
  KEY `is_active` (`is_active`)
) ENGINE=InnoDB AUTO_INCREMENT=2020 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;