Wordpress: custom content shows before $content

Question

I'd like to show some custom content after the_content. However, I can't make it work. the custom content always shows before the_content. Here's my code:

function after_content($content){



function custom_content() {


include('page.php');
...

 }


$content = $content . custom_content();
return $content;
}

add_filter('the_content', 'after_content');