uninitialized constant Rails in Tests only

Hi Guys,

I can start my server in development mode.

However, when I run rake test. I have the following errors:

config/environment.rb:4: uninitialized constant Rails (NameError)
  from ./test/unit/../test_helper.rb:2:in `require'
  from ./test/unit/../test_helper.rb:2
  from test/unit/course_test.rb:1:in `require'
  from test/unit/course_test.rb:1

Line 4 of environment.rb:
Rails::VERSION::STRING >= '2.3.4' unless defined?
Rails::VERSION::STRING

Line 2 of test_helper.rb
require File.expand_path(File.dirname(__FILE__) + "/../config/
environment")

How can I solve it?

Thanks,
Arthur