In the all forum overview replies show from Anonymous

Just upgraded from Drupal 6.2 to 6.3 and also updated Advanced Forum to 6.x-1.0-alpha2
On the main forum overview page (/forum) the last post column works correctly for new topics, but for replies is shows them as being made by "Anonymous"

I've tracked it down to these lines of code starting on line 502

 if (!empty($variables['topic']->reply_author_id)) {
      $author->uid = $variables['topic']->reply_author_id;
      $author->name = $variables['topic']->reply_author_name; 

It seems that reply_author_id has no value even when there is a reply. So the statement fails. I'm guessing that reply_author_id is not the right variable name, but I am not sure what it should be.