I am trying to create a new rails project in edge:

rails campus
cd campus
rake rails:freeze:edge
script/generate model topic title:string body:text
rake db:create:all
rake db:migrate
script/generate controller topics index new create
rm public/index.html 

I added this line to routes.rb:
map.root :controller => "topics"

I type http://localhost:3000 and get this message:
Status: 500 Internal Server Error Content-Type: text/html
In the development.log I see this:
Status: 500 Internal Server Error
A secret is required to generate an integrity hash for cookie session data. Use config.action_controller.session = { :session_key => "_myapp_session", :secret => "some secret phrase" } in config/environment.rb

what am I doing wrong?