Shortcode giving headers already sent message

I have a shortcode used to display posts from a certain custom post type. Initially the shortcode output was showing above the_content and with some googling I found that I needed to use the ob_start, ob_end, etc. This made it show where I wanted it, wherever in my post. However, I now get the following error message and am not sure how to fix it:
Warning: Cannot modify header information – headers already sent by (output started at /home/myaccount/wp-content/themes/mythemename/includes/shortcodes/shortcode-posts.php:47) in /home/myaccount/wp/wp-includes/pluggable.php on line 876

This is the shortcode php:

<?php

function mythemename_posts( $atts, $content = null) {

	extract(shortcode_atts(array(
		'post_type' => 'posttypename'
		), $atts));

	$args = array(
		'post_type' => $post_type,
		'showposts' => -1
	);

	ob_start();
	?>

	<?php
	$custom_loop = new WP_Query( $args );
  if ( $custom_loop->have_posts() ) :
  ?>

  <ul>
  <?php while( $custom_loop->have_posts() ) : $custom_loop->the_post(); ?>
  <li>
    <a href="<?php the_permalink(); ?>">
  		<?php if ( has_post_thumbnail()) : // Check if Thumbnail exists ?>
  		  <?php the_post_thumbnail(); // Fullsize image for the single post ?>
  		  <span></span>
  	  <?php else: ?>
  	    <span class="no-photo"></span>
  		<?php endif; ?>
  		<?php the_title(); ?>
  	</a>
  </li>
  <?php endwhile; ?>
  </ul>

  <?php endif; ?>
  <?php wp_reset_query(); ?>

  <?php $output_string = ob_get_contents(); ?>
  <?php ob_end_clean(); ?>
  <?php return $output_string; ?>

  <?php } ?>

<?php add_shortcode('listposts', 'mythemename_posts'); ?>