SET NULL Doesn't Work

I have this table:
MySQL
CREATE TABLE `users` (
  `id` INT(11) NOT NULL AUTO_INCREMENT,
  `login` VARCHAR(20) NOT NULL,
  `password` VARCHAR(20) NOT NULL,
  `name` VARCHAR(30) NOT NULL,
  `email` VARCHAR(30) NOT NULL,
  `icq` INT(9) UNSIGNED DEFAULT NULL,
  `personal_stream_id` INT(11) NOT NULL,
  `TIME` DATETIME NOT NULL,
  `url_name` VARCHAR(30) DEFAULT NULL,
  `avatar` VARCHAR(20) DEFAULT NULL,
  PRIMARY KEY  (`id`),
  UNIQUE KEY `id` (`id`),
  UNIQUE KEY `login` (`login`),
  UNIQUE KEY `email` (`email`),
  UNIQUE KEY `icq` (`icq`),
  CONSTRAINT `users_fk` FOREIGN KEY (`id`) REFERENCES `photos` (`user_id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=INNODB DEFAULT CHARSET=latin1;

The constraint is linking this table:


MySQL
CREATE TABLE `photos` (
  `id` INT(11) NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(20) NOT NULL,
  `user_id` INT(11) DEFAULT NULL,
  `streams_id` INT(11) NOT NULL,
  `description` TEXT,
  `hardware_id` INT(11) DEFAULT NULL,
  `ratings_id` INT(11) DEFAULT NULL,
  `thread_id` INT(11) DEFAULT NULL,
  `url_name` VARCHAR(20) DEFAULT NULL,
  `views` INT(11) DEFAULT NULL,
  PRIMARY KEY  (`id`),
  UNIQUE KEY `id` (`id`),
  KEY `ratings_id` (`ratings_id`),
  KEY `user_id` (`user_id`),
  KEY `streams_id` (`streams_id`)
) ENGINE=INNODB DEFAULT CHARSET=latin1;

It works, but as you can see, the constraint ondelete action is set to CASCADE, which is not good - it is bad to delete all photos when the user is deleted, I want the database to set user_id value to NULL using the SET NULL action - PHP frontend should display something like "User deleted" then. But when I try to alter the constraint to SET NULL, EMS SQL Manager I'm using shows a very strange error like "Can't create table '.dfk#sql-8e4_4.frm' (errno: 150)". Similiar error is shown when I try to link unsigned int with not unsigned int.