MySQL FOREIGN KEY constraint is incorrectly formed

I have the following table definition: CREATE TABLE async_task ( idasync_task bigint(20) unsigned NOT NULL AUTO_INCREMENT, idasync_type int(10) unsigned NOT NULL, priority tinyint(3) NOT NULL, status enum('todo','doing','failed') NOT NULL DEFAULT 'todo', iduser int(11) NOT NULL, date_added datetime NOT NULL, PRIMARY KEY (idasync_task), KEY priority_id (priority,idasync_task), KEY status_type (status,idasync_type) ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8; I've added a column to the my notification table which I want to point to async_task: ALTER TABLE notification ADD COLUMN async_task_id BIGINT(20) And when I add the following foreign key: ALTER TABLE notification ADD CONSTRAINT fk_notification_async_task FOREIGN KEY (async_task_id) REFERENCES async_task(idasync_task); I get: ERROR 1005 (HY000): Can't create table my_database.#sql-182_2d (errno: 150 "Foreign key constraint is incorrectly formed") I've looked elsewhere but only find the errors as being: 1. The table you're referencing is not created (not the case) 2. The table you're referencing is not InnoDB (not the case, both notification and async_task are InnoDB) 3. You're not referencing the entire primary key (not the case, the only primary key is the ID column). What else could it be?