Page number in viewtopic page title is incorrect

http://area51.phpbb.com/phpBB/viewtopic.php?f=99&t=41639&start=100 that's page 11, but the title says 6.

The value is being calculated using the topics_per_page value instead of posts_per_page.

$pagination->get_on_page($config['topics_per_page'], $start)