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