Unable to resolve controller for.. in ControllerTest

[Standa Mach]

I am using the code below to test my controller.

If run like this, it produces the error listed below.
If, however, I uncomment the test_index method, everything works just fine.

I am using ruby-...@rails4.0.0
...............................................................................................
require "test_helper"

class StudentsControllerTest < ActionController::TestCase
  include Devise::TestHelpers

  before do
    @student = FactoryGirl.create(:student)
  end

  #def test_index
  #end

  describe "stranger" do
    test "can access index" do
      get :index
      assert_response :success
      assert_not_nil assigns(:students)
    end
  end

  describe "user" do
  end
end

................................................................
test_can_access_index
        Unable to resolve controller for user
        Exception `NameError' at:
        /home/stanley/.rvm/gems/ruby-2.0.0-p247@rails40/gems/actionpack-4.0.0/lib/action_controller/test_case.rb:438:in `controller_class'
        /home/stanley/.rvm/gems/ruby-2.0.0-p247@rails40/gems/actionpack-4.0.0/lib/action_controller/test_case.rb:589:in `setup_controller_request_and_response'
        /home/stanley/.rvm/gems/ruby-2.0.0-p247@rails40/gems/activesupport-4.0.0/lib/active_support/callbacks.rb:387:in `_run__3961909968890346606__setup__callbacks'
        /home/stanley/.rvm/gems/ruby-2.0.0-p247@rails40/gems/activesupport-4.0.0/lib/active_support/callbacks.rb:80:in `run_callbacks'
        /home/stanley/.rvm/gems/ruby-2.0.0-p247@rails40/gems/activesupport-4.0.0/lib/active_support/testing/setup_and_teardown.rb:26:in `before_setup'
        /home/stanley/.rvm/gems/ruby-2.0.0-p247@rails40/gems/activerecord-4.0.0/lib/active_record/fixtures.rb:713:in `before_setup'
................................................................
Gemfile:

group :development, :test do
  gem 'minitest-rails'
end

group :development do
  gem 'guard'
  gem 'guard-minitest'
end

group :test do
  gem 'spring'
  gem 'minitest-rails-capybara'
end

gem 'factory_girl'
gem 'turn'
gem 'devise'