How can I limit the number of posts in tag page?

Hello

I added the following function to limit the number of posts in tag archive page, but it does not work:

add_filter('pre_get_posts', 'limit_change_tag_archive');
function limit_change_tag_archive($query){
    if ($query->is_tag) {
        $query->set('posts_per_page', 7);
    }
    return $query;
}

How can I limit the number of posts in tag page?