NoMethodError: undefined method `update_average_rating' for nil:NilClass

Question

I have a rail application with 3 models.

class User < ActiveRecord::Base
  has_many :ratings
end

class Movie < ActiveRecord::Base
  has_many :reviews
  accepts_nested_attributes_for :reviews

end

class Review < ActiveRecord::Base
belongs_to :movie
belongs_to :user
after_create :update_movie_rating
  def update_movie_rating
    movie.update_average_rating
  end
end

I'm trying to update the average rating of movie after creating each reviews.

Here is the methods I use in movie model.

def update_average_rating(review=nil)
  s = self.reviews.sum(:rating)
  c = self.reviews.count
  self.update_attribute(:average_rating, c == 0 ? 0.0 : s / c.to_f)
end

This method is using with after_create callback in the review model.

But I get error like this when I deploy to heroku. I can't make post to the the application which is deployed on heroku. What will be the reason behind it? I get it right while trying to update it with admin panel (active admin).

NoMethodError (undefined method `update_average_rating' for nil:NilClass):
2015-06-08T20:09:06.297312+00:00 app[web.1]:   app/models/review.rb:6:in `update_movie_rating'
2015-06-08T20:09:06.297314+00:00 app[web.1]:   app/controllers/reviews_controller.rb:36:in `block in create'
2015-06-08T20:09:06.297315+00:00 app[web.1]:   app/controllers/reviews_controller.rb:35:in `create'

How can I solve it?