Get value of checkbox after user registration

Hi

I've got this:

add_action("gform_user_registered", "process_order", 10, 4);

function process_order($user_id, $config, $entry, $user_pass) {
    // get first and last name from $entry
    $order_id = $entry['id'];
    $quantity = $entry['8'];
    $refresher = $entry['15'];
    $tx = $entry['transaction_id'];
}

$entry['15'] is a checkbox field. I can successfully get the value of the other fields but not of the checkbox - is there something different I need to do for that type of field?