Rails4 NoMethodError (undefined method `[]' for nil:NilClass):

Hello I'm getting the error below by using Rails 4. My front-end(EmberJs) provides json.

Started POST "/articles" for 127.0.0.1 at 2014-03-30 00:54:18 -0400 Processing by ArticlesController#create as JSON Parameters: {"article"=>{"title"=>"Title", "body"=>"Content", "author_id"=>"2", "author"=>nil}} Unpermitted parameters: author Completed 500 in 4782ms

NoMethodError (undefined method []' for nil:NilClass): app/controllers/articles_controller.rb:43:inblock in create' app/controllers/articles_controller.rb:42:in `create'

My Model looks like this:

class Article < ActiveRecord::Base
 validates :title, presence: true
 validates :body, presence: true

 attr_accessible :title, :body, :author_id

 belongs_to :author
end

My controller looks like this:

class ArticlesController < ApplicationController
    def new
      @article = Article.new
      respond_to do |format|
       format.json { render json: @article }
      end
    end

   def create
    @article = Article.new(article_params)

    respond_to do |format|
      if @article.save
        format.json { render json: @article, status: :created, location: @article }
      else
        format.json { render json: @article, status: :unprocessable_entity }
      end
    end
  end

  private
  def article_params
    params.require(:article).permit(:title, :body, :author_id)
  end
end