Problem with avatar into a function

Hi,

I programmed a custom function calling the recent comments of my blog (https://onirisweb.net/category/blog/) that I placed in my sidebar.

My matter is that I can’t succeed to call the custom avatars of the authors, just only the default one.

Here is my code :

<?php
  $pre_HTML ="";
  $post_HTML ="";
  global $wpdb;
  $sql = "SELECT DISTINCT ID, post_title, post_password, comment_ID, comment_post_ID, comment_author, comment_date_gmt, comment_approved, comment_type,comment_author_url, SUBSTRING(comment_content,1,60) AS com_excerpt FROM $wpdb->comments LEFT OUTER JOIN $wpdb->posts ON ($wpdb->comments.comment_post_ID = $wpdb->posts.ID) WHERE comment_approved = '1' AND comment_type = '' AND post_password = '' ORDER BY comment_date_gmt DESC LIMIT 5";

  $comments = $wpdb->get_results($sql);
  $output = $pre_HTML;
  $output .= "\n<ul>";
  foreach ($comments as $comment) {
    $output .= "\n<li>".get_avatar( $comment->comment_author_email, 50)."?<strong>".$comment->post_title . "</strong>?".strip_tags($comment->comment_author)."?a dit :<br/><a href=\"" . get_permalink($comment->ID)."#comment-" . $comment->comment_ID . "\" title=\"on ".$comment->post_title . "\">" . strip_tags($comment->com_excerpt)."?[...]</a></li>";
  }
  $output .= "\n</ul>";
  $output .= $post_HTML;
  echo $output;
?>

I don’t understand why the good one can’t be displayed.

Anyone could tell me where is the problem ?

Thanks for your help