Mysql - Why can i not cast varchar to longtext?

Question

I can't seem to get my cast to work correctly

Below is my sample code

CREATE TABLE test (
  id int UNSIGNED NOT NULL AUTO_INCREMENT,
  refname varchar(40) NOT NULL,
  parent_id int UNSIGNED DEFAULT NULL,
  is_active int UNSIGNED DEFAULT 1,
  PRIMARY KEY (id),
  FOREIGN KEY (parent_id) REFERENCES test (id)
    ON DELETE CASCADE ON UPDATE CASCADE
);

  SELECT id, refname, parent_id, 1 AS depth, CAST(refname as LONGTEXT)
    FROM test
    WHERE parent_id IS NULL

Whenever i try to run this query it gives me

Error Code: 1064. You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'LONGTEXT) FROM test WHERE parent_id IS NULL' at line 1

I have gone though several resources to try and identify what is wrong here, But I cannot seem to figure this out.

Does anyone have any ideas?

Thanks all :)