Featured Image on Post page

Hi,

I’m building a static website. On Settings > Reading I’ve selected a “front page” and a “post page”. And now I want to show a “featured image” on all pages as a part of the header and this works on all pages but the “post page”

This is the code I use
<?php if ( has_post_thumbnail() ) { the_post_thumbnail( 'header-image' ); } ?>

I want to show the “post page” image and not the feature image from the first post!

Thanks for the help.