MEDIUMTEXT field

I have a field which I want to be mysql mediumtext
/**

• @var string $description
• @ORM\Column(name="description", type="string", length=65536)
  */
  private $description;

Everything is fine, field created as 'mediumtext'
but whenever I run
php app/console doctrine:schema:update --dump-sql

I get
ALTER TABLE table CHANGE description description MEDIUMTEXT NOT NULL;
though description is MEDIUMTEXT already.