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=

\n Posted less than a minute \nago</
\n rails \n

\n\n” });

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