Fatal error: Call to undefined function block_class() in /home/admin/public_html/themes/hammerhead/block.tpl.php on line 4

Can you please help

here is how the code looks like and it gives me error.

<?php
// $Id: block.tpl.php,v 1.3 2007/08/07 08:39:36 goba Exp $
?>
<div id="block-<?php print $block->module .'-'. $block->delta; ?>" class="clear-block block block-<?php print $block->module ?> <?php print block_class($block); ?>">


<?php if (!empty($block->subject)): ?>
  <h2><?php print $block->subject ?></h2>
<?php endif;?>

  <div class="content"><?php print $block->content ?></div>
</div>