With null_session, cookies.signed raises exception

[js1888]

In Edge Rails (443be75), using cookies.signed with a null_session raises "TypeError (can't convert nil into String)" at "rails/activesupport/lib/active_support/key_generator.rb:21:in `pbkdf2_hmac_sha1'".

# application_controller.rb
class ApplicationController < ActionController::Base
  protect_from_forgery with: :null_session
  def index
    cookies.signed[:foo] = 'foo'
  end
end

# routes.rb
Railstest::Application.routes.draw { post '/' => 'application#index' }

$ curl -d mydata http://localhost:3000

[a-chernykh]

I've tried to reproduce your case, but :null_session with signed cookies works just fine for me. I've tried with the following Gemfiles:

source 'https://rubygems.org'
gem 'rails', :git => 'git://github.com/rails/rails.git'

source 'https://rubygems.org'
gem 'rails', :git => 'git://github.com/rails/rails.git', :ref => '443be75'

[js1888]

I just reproduced it with these steps. Here's the response and stack trace. Here's my Gemfile and Gemfile.lock. Anything look wrong or weird with my setup?

[spastorino]

I've removed the CHANGELOG entry b5645d0 because there's no need to provide that for a feature that was not released yet.
Sorry for making you go back and forth.