我已按照http://railscasts.com/episodes/235-omniauth-part-1和http://railscasts.com/episodes/236-omniauth-part-2的教程进行操作，但出现错误
You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.[]
我应该怎么办？
更新：错误
app/models/user.rb:9:in `apply_omniauth'
app/controllers/authentications_controller.rb:18:in `create'