Issue with rewrite.php

I’m getting the error:
Warning: in_array() [function.in-array]: Wrong datatype for second argument in /homepages/33/d236206946/htdocs/fbcwaynesboro.org/www/wp/wp-includes/rewrite.php on line 59
on my page
Looking at the referenced code, I can’t quite figure out how to modify it to remove the warning:
lines 57-61 in rewrite.php

function add_feed($feedname, $function) {
	global $wp_rewrite;
	if (!in_array($feedname, $wp_rewrite->feeds)) { //override the file if it is
		$wp_rewrite->feeds[] = $feedname;
	}