ExtJS Ajax Request and Rails Authenticity Token

I was testing out ExtJS with Rails 2.1. As I was making a POST request to the server, I get an ActionController::InvalidAuthenticityToken. It's bugging me for a little while until I get to this site. I made my own fix and put the following code in the header part of my layout after ExtJS has been loaded:

Ext.Ajax.extraParams = { authenticity_token: "<%= form_authenticity_token %>" };

And everything is working for me.