config.action_controller.session_options[:cookie_only] = true

Recently, I got this security vulnerability on my app:

Ruby on Rails Multiple Method Session Fixation
      Synopsis :

      The remote web server is affected by a session fixation
vulnerability.

      Description :

      The web server on the remote host appears to be a version of
Ruby on
      Rails that supports URL-based sessions. An unauthenticated
remote
      attacker may be able to leverage this issue to obtain an
authenticated
      session.

      Note that Ruby on Rails version 1.2.4 was initially supposed to
      address this issue, but its session fixation logic only works
for the
      first request, when CgiRequest is first instantiated.

      See also :

      http://weblog.rubyonrails.org/2007/10/5/rails-1-2-4-maintenance-release
      http://www.nessus.org/u?2f5b72e6
      http://dev.rubyonrails.org/ticket/10048
      http://www.nessus.org/u?1eeea9de

      Solution :

      Upgrade to Ruby on Rails version 1.2.6 or later and make sure
      'config.action_controller.session_options[:cookie_only]' is set
to
      'true' in the 'config/environment.rb' file.

I checked my rails version: it is already 1.2.6. Then I un-comment
this line in environment.rb
config.action_controller.session_options[:cookie_only] = true

I got the following:

Error message:
    You have a nil object when you didn't expect it! You might have
expected an instance of Array. The error occurred while evaluating nil.
[]=
Exception class:
    NoMethodError

Can anyone help me understand what is going on here?