[MYSQL]foreign key problem

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Ciao, CON MYSQL

CREATE TABLE users (
    user_id INT NOT NULL PRIMARY KEY,
    first_name CHAR(32),
    last_name CHAR(32),
    zip_code INT,
    phone INT
    )ENGINE=INNODB;

CREATE TABLE zip (
    zip_code INT,
    city_id INT,
    PRIMARY KEY(zip_code)
    )ENGINE=INNODB; 

poi ho fatto

ALTER TABLE users ADD FOREIGN KEY (zip_code) REFERENCES zip(zip_code) ON DELETE CASCADE ON UPDATE CASCADE; 

e cercando di inserire i dati mi da errore

INSERT INTO users VALUES(1, Franco', 'Rossi', 11100, 123456);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`example`.`users`, CONSTRAINT `users_ibfk_1` FOREIGN KEY (`zip_code`) REFERENCES `zip` (`zip_code`) ON DELETE CASCADE ON UPDATE CASCADE) 

Quale è il problema?