[Plugin: Yet Another Related Posts Plugin] Call to a member function have_posts() on a non-object er

I am trying to include YARP in my single.php page. This is my code:

<div class="similar_posts">
	<h3>Related Posts</h3>					

	<?php if ($related_query->have_posts()):?>
		<ul>

		<?php while ($related_query->have_posts()) : $related_query->the_post(); ?>
			<li><a href="<?php the_permalink() ?>" rel="bookmark"><?php the_title(); ?></a></li>
		<?php endwhile; ?>
		</ul>

	<?php else: ?>
		<p>No related posts.</p>
	<?php endif; ?>					

</div>

I am getting a “Call to a member function have_posts() on a non-object error on line 31” which refers to <?php if ($related_query->have_posts()):?>

Where is this $related_query object declared or created? Would appreciate any help.