Removing the last item of a cart

[eluser]Блум[/eluser]
As it is written in http://ellislab.com/codeigniter/user-gui.../cart.html :

Code:

$data = array(
            'rowid' => $row_id,
            'qty' => 0
);
$this->cart->update($data);

should remove it. But it doesn't. :/ Talking about the last single item in the cart.