404 & wrong datatype for array

I’ve got a function that finds out if the current page is a sub page of another. Lifted the code from a post here I think:

<?php // detect the page tree and set the stylesheet accordingly
function is_tree($p_id) {
	global $post;
	if(is_page()&&(in_array($p_id, $post->ancestors) ||
	   $post->post_parent==$p_id ||
	   is_page($p_id)))
	return true;
	else return false;
};
?>

This lets me add a style sheet according the the top level parent. Anyway. When testing the 404 page the header goes wrong:

Warning: in_array() [function.in-array]: Wrong datatype for second argument in /home/fhlinux133/i/inq-chat.duttonsdev.co.uk/user/htdocs/inq_cms/wp-content/themes/chat_combined/desk_header.php on line 39

with line 39 being the 3rd line of code above.

This error is repeated 8 times.

Any ideas?