How to validate hidden field

Hi All,

I have a multi-page webform with 1 component shown on every page, so there's a pagebreak in between every component, with the exception of the 1st page. On the first page I added a hidden field that gets populated from the url parameter (i.e http://www.example.com/node/10?serialno=001, the ?serialno=001 part of the URL is added programmatically before the complete URL is sent to the user) and the default value of the hidden field is set as %get[serialno]. This works fine most of the time but some time I'm getting empty serialno (from my webserver log, somehow the query string is missing from the URL but that's another story). So just to be safe, I want to make sure that user can only submit if the hidden field is not an empty string. Apparently, the Mandatory checkbox is set by default for hidden field but it doesn't seems to affect hidden field at all. I've tried adding the following to the Additional Validation of the form but it always said the hidden field is empty.

<?php
   $serialno = $form_values['submitted_tree']['serialno'];
   if ($serialno == "") {
     form_set_error('submitted][serialno', t('Missing Serial#, please make sure you use the provided URL.'));
   }
?>

Can anyone help me out? Thanks in advance.