WordPress body classes

gistfile1.php

//print page and parent slugs for css classes
function page_slug_classes($classes) {
  global $post;
	$classes = array();

	//get the slug of any parent pages
	$parent_slug = basename(get_permalink($post->post_parent));
	array_push($classes, $parent_slug);

	//get the slug of the current page
	if ( is_page() && $post->post_parent ) {
		$page_slug = basename(get_permalink());
		array_push($classes, $page_slug);
	}

	return $classes;
}
add_filter('body_class','page_slug_classes');

Having this in my functions.php results in no body classes. However, if I call the function directly and use print_r, it prints the array with the correct slugs in it.