deviseの複数モデルでのストロングパラメーターの実装がわかりません。

こんにちは。

【環境】
gem 'rails', '4.2.5'
devise (4.2.0)

【やりたいこと】

deviseを使った複数モデルでのsign_upの実装。

【現状】

以下の記事を参考にdeviseでモデルとコントローラーを作成しました。
http://qiita.com/Yama-to/items/54ab4ce08e126ef7dade

https://second-app-sagae.c9users.io/users/sign_upページで
sign upボタンを押すと

Email can't be blank
Password can't be blank
Username can't be blank

というエラー表示がでて先に進めません。
ちなみに、validationはusername以外はつけてません。

原因がわからなく大変困っております。
registrations_controller.rbのコードは以下です。

class Users::RegistrationsController < Devise::RegistrationsController
  before_action :configure_sign_up_params, only: [:create]
  #before_filter :configure_permitted_parameters, if: :devise_controller?

# before_action :configure_account_update_params, only: [:update]

  # GET /resource/sign_up
  #def new
    #@user = User.new
  #end

  # POST /resource
  def create
    @user = User.new
    @user = User.new(configure_sign_up_params)
    if @user.save
      flash[:success] = "Welcome to the Sample App!"
      redirect_to @user
    else
      render 'new'
    end
  end

  # GET /resource/edit
  #def edit
    #super
  #end

  # PUT /resource
  #def update
    #super
  #end

  # DELETE /resource
  #def destroy
   # super
  #end

  # GET /resource/cancel
  # Forces the session data which is usually expired after sign
  # in to be expired now. This is useful if the user wants to
  # cancel oauth signing in/up in the middle of the process,
  # removing all OAuth session data.
  #def cancel
    #super
  #end

  # protected

  # If you have extra params to permit, append them to the sanitizer.
  def configure_sign_up_params
    devise_parameter_sanitizer.permit(:sign_up, keys: [:username])
  end
  

  # If you have extra params to permit, append them to the sanitizer.
  #def configure_account_update_params
    #devise_parameter_sanitizer.permit(:account_update, keys: [:attribute])
  #end

  # The path used after sign up.
  #def after_sign_up_path_for(resource)
    #super(resource)
  #end

  # The path used after sign up for inactive accounts.
  #def after_inactive_sign_up_path_for(resource)
    #super(resource)
  #end
  
end