I have a dropdown box enabled on my checkout page. When a customer hasn't selected something from the dropdown, I need to redirect them to the checkout payment page and show them an error message.

Here's what I have put for code in my checkout_confirmation header.php:

Code:
if (!isset($_SESSION['dropdown'])) {
  
$messageStack->add_session('dropdown', ERROR_DROP_DOWN, 'error'); 

zen_redirect(zen_href_link(FILENAME_CHECKOUT_PAYMENT, '', 'SSL'));
  
}
It kicks it back to the payment page, but doesn't show the error message. Can anyone help me correct where I've gone wrong?