In the function uc_zone_select() you need to modify it to look like the following:
<?php
$select = array(
'#type' => 'select',
'#title' => $title,
'#description' => $description,
'#options' => $options,
'#default_value' => $default,
'#required' => TRUE, // <--- Add this line.
);
?>
Joined: 08/07/2007