Nil class error for association

I have a model Tutorial and has_many association Tutorialcategory where tutorials can be connected to many tutorial categories. Tutorials model and controller are working fine but categories are not. I am able to add categories but can not edit them, I am getting 'undefined method `model_name' for NilClass:Class' error while accessing edit page for tutorialcategory.

I have defined route as follows

resources :tutorialcategories do
    end

following is my model definition

class Tutorial < ActiveRecord::Base
    has_many :tutorialcategories