tests , assertion output value always <>

no problem running my tests ( units , functionals ) but when assertion failed , I never see any assertion output value, it's always <> and expected also is <>

i.e. :

  => 33: assert ability.can? :index, User      34: get :index      35: assert_response :success      36: assert_not_nil assigns(:users) <> expected but was <>

I must have some wrong parameter , but where ? any clue welcome..

group :test do   gem 'single_test'   gem 'simplecov'   gem 'fakeweb'   gem 'autotest-rails' end