Can not add foreign key constraints

Hello,

 I'm trying to add a table to an existing database. I'm wanting it to get
 one of it's fields from an already existing table. I've done this
 before in this database. This works:

 CREATE TABLE `virtual_users` (
   `id` int(11) NOT NULL auto_increment,
   `domain_id` int(11) NOT NULL,
   `user` varchar(40) NOT NULL,
   `password` varchar(128) NOT NULL,
   `quota` bigint(20) NOT NULL DEFAULT 256,
   `quota_messages` int(11) NOT NULL DEFAULT 0,
   PRIMARY KEY (`id`),
   UNIQUE KEY `user` (`user`),
   FOREIGN KEY (domain_id) REFERENCES virtual_domains(id) ON DELETE CASCADE
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

 This does not:

 CREATE TABLE IF NOT EXISTS `lastauth` (
 `user` varchar(40) NOT NULL,
 `remote_ip` varchar(18) NOT NULL,
 `timestamp` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE
 CURRENT_TIMESTAMP,
 PRIMARY KEY (`user`),
 FOREIGN KEY (user) REFERENCES virtual_users(user) ON DELETE CASCADE
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

 Can anyone spot the situation?

 Thanks.
 Dave.

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