Always get a default avatar in my comments

Hi,

Since I have installed the 2.5.1 and installed the advised code, I alwys get the default gravatar displayed.
Can anyone help?

The code is in the comments loop after:
<?php foreach ($comments as $comment) : ?>
And looks like this

<div class="gravatar">
   <?php if (function_exists('get_avatar')) {
      echo get_avatar($email, $size='60');
   } else {
      //alternate gravatar code for < 2.5
      $grav_url = "http://www.gravatar.com/avatar.php?gravatar_id=
         " . md5($email) . "&default=" . urlencode($default) . "&size=" . $size;
      echo "<img src='$grav_url'/>";
   } ;?>
   </div>

Thanks