MySQL Error 1452 - Unable to add or update child row: foreign key constraint not fulfilled

Question

I looked at other questions on this topic, but cannot find where my error came from. Any help would be greatly appreciated. I include as much as I can think of, which can help find the problem:

CREATE TABLE stocks (
id INT AUTO_INCREMENT NOT NULL,
user_id INT(11) UNSIGNED NOT NULL,
ticker VARCHAR(20) NOT NULL,
name VARCHAR(20),
rating INT(11),
position ENUM("strong buy", "buy", "sell", "strong sell", "neutral"),
next_look DATE,
privacy ENUM("public", "private"),
PRIMARY KEY(id),
FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
)
 ENGINE=InnoDB DEFAULT CHARSET=utf8;



CREATE TABLE IF NOT EXISTS `stocks_tags` (
  `stock_id` INT NOT NULL,
  `tag_id` INT NOT NULL,
  PRIMARY KEY  (`stock_id`,`tag_id`),
  KEY `fk_stock_tag` (`tag_id`),
  KEY `fk_tag_stock` (`stock_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

ALTER TABLE `stocks_tags`
  ADD CONSTRAINT `fk_stock_tag` FOREIGN KEY (`tag_id`) REFERENCES `tags` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  ADD CONSTRAINT `fk_tag_stock` FOREIGN KEY (`stock_id`) REFERENCES `stocks` (`id`) ON DELETE CASCADE ON UPDATE CASCADE;



CREATE TABLE tags(
id INT AUTO_INCREMENT NOT NULL PRIMARY KEY,
tags VARCHAR(30),
UNIQUE(tags)
)
ENGINE=INNODB DEFAULT CHARSET=utf8;

And the error I get:

Database_Exception [ 1452 ]: Cannot add or update a child row: a foreign key constraint 
fails (`ddmachine`.`stocks_tags`, CONSTRAINT `fk_stock_tag` FOREIGN KEY (`tag_id`) REFERENCES  
`tags` (`id`) ON DELETE CASCADE ON UPDATE CASCADE) [ INSERT INTO `stocks_tags` (`stock_id`, 
`tag_id`) VALUES (19, 'cash') ]

I saw that someone had a similar problem based on their listing columns, but don't think that it is.

Answer 1

This means that tag_id is cashnot in the tag table ... should tag_id not be an integer?

(in this question I mean)

INSERT INTO `stocks_tags` (`stock_id`, `tag_id`) VALUES (19, 'cash')