add foreign key that ref's column in same table (7 replies)

I'm trying to add a foreign key that references the primary id in the same table.

But when I do so, all I get is a message that " #1215 - Cannot add foreign key constraint".

This is my table definition and the code I'm trying to use to add the fk is below that.

CREATE TABLE IF NOT EXISTS `categories` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`name` varchar(48) NOT NULL,
`userID` smallint(5) unsigned NOT NULL,
`type` enum('income','expense') NOT NULL,
`parentID` bigint(20) unsigned DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=102 ;


ALTER TABLE `categories`
ADD KEY `fkParentID` (`parentID`),
ADD CONSTRAINT `fkParentID` FOREIGN KEY (`parentID`)
REFERENCES `categories` (`id`) ON DELETE CASCADE ON UPDATE CASCADE