Rails 2 blog screencast, ajax error

I've been following the '15 minute blog ' screencast on the rails site. Got to the part when you add ajax functionality to the comments form and when i try to post a new comment this way in Firefox, i get an alert window with the following message:

RJS error:

TypeError: null has no properties

and then, when i click 'ok':

Element.insert("comment", { bottom: "<div class=\"comment\" id= \"comment_12\">\n <p>\n <strong>Posted less than a minute \nago</

<br />\n rails \n </p>\n</div>\n" });

$("comment_12").visualEffect("highlight"); $("new_comment").resetlocal();

I'm pretty sure there are no typos in the code, I'm new to rails so any help is welcome. Thanks in advance