Can't get sessions to work. Getting 500 Internal Server Error.

I've been fiddling around with rails for a while now. Following
books, tutorials etc.
But as soon as I tried to use sessions in my application, eg. by
calling session[:user] = 'something'
All I get is "500 Internal Server Error"

(Both in webrick and mongrel) on OS X Leopard using ruby 1.8.6
I've also tried upgrading from rails 2.0.2 to 2.1.0


From ApplicationController:

def initialize
session[:test] = 'something'

It shouldn't be anything wrong with this as far as I know.

I'm afraid there is something wrong with that: you're overriding
ActionController's initialize one with your own, with doesn't call the
super class implementation, probably doesn't have the same signature,
and at that point the session isn't setup.
What are you trying to do?