wp_list_pages – why dont the child menu’s indent?

Why do the child pages not indent under the parent pages in this list?

<div id="sidebar">
<ul>
<?php if ( !function_exists('dynamic_sidebar')
        || !dynamic_sidebar() ) : ?>

<ul>
<?php
wp_list_pages('sort_column=menu_order&depth=0&title_li=');
?>
</ul>
<?php endif; ?>
</div>