Call super here to allow SessionHash initialize new stuff for us

actionpack/lib/action_controller/test_case.rb

@@ -210,7 +210,7 @@ module ActionController
     DEFAULT_OPTIONS = Rack::Session::Abstract::ID::DEFAULT_OPTIONS
 
     def initialize(session = {})
-      @env, @by = nil, nil
+      super(nil, nil)
       replace(session.stringify_keys)
       @loaded = true
     end