Problem with wp_list_pages()

I included this in in my sidebar:

<ul>
<?php wp_list_pages('title_li='); ?>
</ul>

The list appeared on the main page of the blog, no problems other than bad style. But when I clicked on the child link (second page), I got this error:

Warning: Invalid argument supplied for foreach() in /home/crowspea/public_html/wp-includes/template-functions-post.php on line 368

I have since removed the the code. There are currently only two pages (I plan to add more), and I am not using a different template other than my default theme template.

Anyone see this before?

Crow