PHP: WordPress Parent Featured Image

get_featured_recursive.php

<?php
// recursively looks for featured images
function get_featured_recursive( $post ) {
	if ( has_post_thumbnail( $post->ID ) ) {
		return $post->ID;
	} else if ( $post->post_parent != 0 ) {
		return get_featured_recursive( get_post( $post->post_parent ) );
	} else {
		return null;
	}
}


// display on template
$featured_image_post = get_featured_recursive( $post ); 

if ( $featured_image_post != null ) {
	$featured_image_src = wp_get_attachment_image_src( get_post_thumbnail_id( $featured_image_post ), 'single-post-thumbnail' ); 
}
?>
<img src="<?php echo $featured_image_src[0]; ?>" alt="">

This no longer appears to work under Wordpress 3.5 - any ideas why?