Error in wp-commentsrss2.php

Hi,
When I look through my error logs on my hosting package I get quite a lot of this:
[Thu Dec 9 09:14:26 2004] [error] PHP Warning: Invalid argument supplied for foreach() in /home/myusername/public_html/wp-commentsrss2.php on line 16
When I llok at the file in a browser it seems fine. I have never edited this file so am unsure why it’s happening.
Line sixteen is:
foreach ($posts as $post) { start_wp();
Does anybody have any idea what’s wrong?
TIA
Phillip