Help me add a "alt" attribute in this code.

I want that my gravatars have the “alt” attribute in the image. what should i change? this is my template code:

<div class="comment-author vcard">
         <?php echo get_avatar($comment,$size='50',$default='' ); ?>
		<a href="<?php echo htmlspecialchars( get_comment_link( $comment->comment_ID ) ) ?>"> <?php comment_date('n-j-Y'); ?></a>
      </div>

thanks !