Topic: rspec, underfine method assert_difference

Hi everyone, I using rails 3.1.3 and I use rspec to test some function
when I using assert_difference to test sum of User change, It have this error.
My code:

it 'call user#update_user_point' do
      assert_difference(@sponge.total_points) do
           event = Event.create user: @sponge, event_type: event_type, points_earned: event_type.points_value, description: {}
      end
end

Help me find out this error. thks big_smile