WordPress function wp_list_pages not accepting "exclude" argument

Hi,

I am trying to use the function wp_list_pages() to generate a simple site map, with a couple of pages excluded from the list, but the exclude argument doesn’t seem to be working. Am I doing something wrong? The list generates fine, and accepts other arguments, it just seems to be ignoring “exclude”.

Here is the code i am using:

<ul>
	<?php
	$args = array(
	 'title_li' => '',
	'exclude' => '4,6',
	'sort_column'  => 'post_title'
	);
       wp_list_pages($args);
	?>
</ul>

Can anyone see where I am going wrong and help? Thanks.

John