Comments callback error

Hello everyone.

I am creating my first WordPress theme. I have added the loop to my index.php and the posts are displaying as they should. However, the comments are not. The number of comments, the link to post a comment and the comment form display as they should, but when you submit a comment I get this error:

Warning: call_user_func(twentyeleven_comment) [function.call-user-func]: First argument is expected to be a valid callback in /home/berrycon/public_html/sandbox/wp-includes/comment-template.php on line 1334

in place of where the comments should show up.

The test site is at https://berryconcept.co.uk/sandbox

The WP code on my index page is as follow:

<?php global $query_string; // required
$posts = query_posts($query_string.'&posts_per_page=3'); // shows only three posts ?>

<!--Start the default loop-->

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

<!-- The following tests if the current post is in category 3. -->
 <!-- If it is, the div box is given the CSS class "post-cat-three". -->
 <!-- Otherwise, the div box will be given the CSS class "post". -->
 <?php if ( in_category('special') ) { ?>
           <div class="post_special">
 <?php } else { ?>

         <div <?php post_class() ?> id="post-<?php the_ID();?>">
 <?php } ?>
<div class="det">
<h1 class="title"><a href="<?php the_permalink()?>"<rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></h1>
 <!-- Display the Title as a link to the Post's permalink. -->
<h2 class="author">by <?php the_author_posts_link() ?></h3>
	<div class="date">
     <h1 class="day"><?php the_time ('d')?></h1>
   	 <h2 class="month_year"><?php the_time('M / y')?> </h2>
	</div><!--end date-->
</div><!--end det-->

 <!-- Display the Post's content (before more tag) in a div box. -->
 <div class="entry"> <?php the_content('<span class="moretext">(more)</span>'); ?>
 <!-- Display a comma separated list of the Post's Categories. -->
<p class="postmetadata">
Posted in <?php the_category(',') ?>
<strong>|</strong>
 <?php edit_post_link('Edit','','<strong>|</strong>'); ?>
</div> <!-- closes entry --> 

<div id="pagecomments">
<?php comments_template();?>
</div>

<div class=comments">
<?php comments_popup_link('There are no comments, please add your thoughts ', 'There is 1 comment, please add your thoughts ', 'There are % comments, please add your thoughts '); ?></p>
</div>

</div><!--closes post or special-->

 <!-- Stop The Loop (but note the "else:" - see next line). -->
 <?php endwhile; else: ?>

 <!-- The very first "if" tested to see if there were any Posts to -->
 <!-- display.  This "else" part tells what do if there weren't any. -->
 <p>Sorry, no posts matched your criteria.</p>
 <?php include("/searchform.php");?>

 <!-- REALLY stop The Loop. -->
 <?php endif; ?>

 <?php wp_reset_query(); // reset the query ?>

I’d really appreciate some help. Many thanks, Tom