When you add an orderby with the meta_key set to the page number, if no value is set, the results are omitted, is there any way around that?

Sample Query

<?php

$args = array(
  'post_type'			=> 'magazine',
	'title'			  	=> 'Articles',
	'post_parent'		=> $post->ID,
	'order'				  => 'asc',
	'meta_key'			=> 'PageNumber',
	'orderby'			  => 'meta_value_num'
	);
make_magazine_toc($args);
?>