wp-list-pages doesn't generate <ul> tags

At http://www.kunstroutelaarbeek.nl/kunstenaars I tried the following piece of code:
<?php wp_list_pages('child_of=324&title_li=&sort_column=menu_order' ); ?>
Since I wanted to remove the bullets in front of it I struggled with various css elements but eventually I noticed (thanks to the w3c validator) that no <ul> and </ul> tags were put around it, as soon as I put them manually with a simple <ul style = "list-style-type:none"> it was solved. I just wonder: is it normal behaviour of wp_list_pages not to generate the <ul> tags? Or did I something wrong with the parameters as shown above? Theme is an elbee-elgee child theme.