Errormessage after search for one post

I get following errormessage when doing a search within my weblog and getting only one result:
Warning: Cannot modify header information – headers already sent by (output started at /home/httpd/vhosts/spoenk.nl/httpdocs/weblog/index.php:2) in /home/httpd/vhosts/spoenk.nl/httpdocs/wordpress/wp-blog-header.php on line 520
This refers to the following content in wp-blog-header.php:
if ($s && empty($paged)) { // If they were doing a search and got one result
if (!strstr($_SERVER['PHP_SELF'], 'wp-admin')) // And not in admin section
header('Location: ' . get_permalink($posts[0]->ID));
}
The page looks good besides this errormessage at the top of the page. The one post is also shown correctly.