Session won't start when using uniqueId

Hi when I use session like that it won't start :

    define('UNIQUE_APP_ID', uniqid());

    $di->set('session', function() {

        $session = new Phalcon\Session\Adapter\Files(
            array(

                'uniqueId' => UNIQUE_APP_ID
            )
        );

        $session->start();

        return $session;
    });

But, when I do this it's working :

    $di->set('session', function() {

        $session = new Phalcon\Session\Adapter\Files();

        $session->start();

        return $session;
    });

Any ideas why ?

Thank you