error in wp-rss2.php

Hello !
I’m having toubles sometimes in wp-rss2.php file
i get this error:

Warning: strtotime() [function.strtotime]: Called with an empty time parameter. in /domain/www_root/wp-blog-header.php on line 138

i didn’t change anything with origin source file.
i see this row in that file:

((strtotime($client_last_modified) >= strtotime($wp_last_modified)) || ($client_etag == $wp_etag)) ) {

i have installed wordpress/1.5.2

anyone can help me with this ?