Check if parent post is sticky on attachment page?

I use this code on post template (single.php) to check if current post is sticky:

<?php
if ( is_sticky() )
echo 'Post is sticky.';
?>

But it is not working on attachment page template (image.php). I wish to check if parent post is sticky on attachment page of post featured image.

How to check if parent post is sticky on attachment page (image.php)?