Underscores.me retrieve next / previous post thumbnail in post_nav function

Question

I solved the problem by using get_the_post_thumbnail() instead of the variables $prevthumbnail and $nextthumbnail.

function THEMENAME_post_nav() {
// Don't print empty markup if there's nowhere to navigate.
$previous = ( is_attachment() ) ? get_post( get_post()->post_parent ) : get_adjacent_post( false, '', true );
$next     = get_adjacent_post( false, '', false );

if ( ! $next && ! $previous ) {
    return;
}
?>
<nav class="navigation post-navigation" role="navigation">
    <!-- <h1 class="screen-reader-text"><?php _e( 'Post navigation', 'THEMENAME' ); ?></h1> -->
    <div class="nav-links">
        <?php
            previous_post_link( '<div class="nav-previous">' . get_the_post_thumbnail() . '%link</div>', _x( '%title', 'Previous post link', 'THEMENAME' ) );
            next_post_link(     '<div class="nav-next">' . get_the_post_thumbnail() . '%link</div>',  _x( '%title', 'Next post link',     'THEMENAME' ) );
        ?>
    </div><!-- .nav-links -->
</nav><!-- .navigation -->
<?php

}