Add Post Tags to Body Class

Seen this successfully done with categories, but for some reason I can't make it work with post tags.

Usage case: Want all posts with tag "course" to have a specific layout and styling applied. Best way I can think of to target them is to target body.tag-XXXX in CSS.

This is the code I've tried, which adds the tags to the article (same happens with generate_main_class), but not to the body.

  function tags_to_body_class( $classes ) {
    global $post;
    $posttags = get_the_tags( $post->ID );

    if ( $posttags ){
        foreach( $posttags as $tag )
            $tags[] = $tag->slug;
        return $tags;
	}
	add_filter('body_class', 'tags_to_body_class');
}