get_next_post returns null

On my single.php i would like to show a “preview” of the next and the previous post.
get_next_post() and get_previous_post() should do the job but they both return Null.

Is there something I’m doing wrong ?

<?php get_header(); ?>

	<section id="portfolio">

	<?php get_next_post(); ?>	

	<?php if ( have_posts() ) : ?>

		<?php while ( have_posts() ) : the_post(); ?>

			<?php get_template_part( 'content-single' ); ?>

		<?php endwhile; ?>

	<?php else : ?>

		<!-- .. -->

	<?php endif; ?>

	<?php get_previous_post(); ?>

	</section>

<?php get_footer(); ?>