Dear stackoverflow, I'm tumbled with a problem!

I've set up my first check constraint using MySQL, but unfortunately I'm having a problem. When inserting a row that should fail the test, the row is inserted anyway.

The structure:

CREATE TABLE user (
  id INT UNSIGNED NOT NULL AUTO_INCREMENT,
  uname VARCHAR(10) NOT NULL,
  fname VARCHAR(50) NOT NULL,
  lname VARCHAR(50) NOT NULL,
  mail VARCHAR(50) NOT NULL,
  PRIMARY KEY (id),
  CHECK (LENGTH(fname) > 3)
);

The insert statement:

INSERT INTO user VALUES (null, 'user', 'Fname', 'Lname', '[email protected]');

I'm pretty sure I'm missing something basic here.