Mysql Foreign Key constraint fails to create with unexisting constraint

Question

So I have two tables, customers and appointments. Im trying to create a very simple FK relation between appointments.customer_id and customers.id, however when I do my ALTER TABLE trying to add the FK im getting this error:

MySQL said: Cannot add or update a child row: a foreign key constraint fails (wax.#sql-2c5_100, CONSTRAINT customer_fk FOREIGN KEY (id) REFERENCES customers (id) ON DELETE CASCADE ON UPDATE CASCADE)

This constraint "#sql-2c5_100" seems to be some randomly generated constraint, that I can not find ANYWHERE. I've looked on every table on the database, ive looked on all the tables on the information schema and it simply does not exist.

Thanks!

Edit: Here's the create table outputs

CREATE TABLE `appointments` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `customer_id` int(11) unsigned NOT NULL,
  `name` varchar(255) DEFAULT NULL
  PRIMARY KEY (`id`),
  KEY `customer_id` (`customer_id`)
) ENGINE=InnoDB AUTO_INCREMENT=290958 DEFAULT CHARSET=utf8;


CREATE TABLE `customers` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `first_name` varchar(255) NOT NULL DEFAULT '',
  `last_name` varchar(255) NOT NULL DEFAULT '',
  `phone` varchar(20) DEFAULT NULL
  PRIMARY KEY (`id`),
  KEY `sf_id` (`sf_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;