我在我的ruby on rails应用程序中使用了doorkeeper来实现OAuth功能，同时我还使用device来实现用户安全管理。
下面是我当前的看门人.rb文件：

Doorkeeper.configure do
  orm :active_record

  resource_owner_authenticator do
    user_signed_in? || redirect_to(new_user_session_url)
  end

  admin_authenticator do
    user_signed_in? || redirect_to(new_user_session_url)
  end

  enable_application_owner :confirmation => true

  # authorization_code_expires_in 10.minutes

  # access_token_expires_in 2.hours

  # use_refresh_token

  # default_scopes  :public
  # optional_scopes :write, :update

  # client_credentials :from_basic, :from_params

  # access_token_methods :from_bearer_authorization, :from_access_token_param, :from_bearer_param

  # test_redirect_uri 'urn:ietf:wg:oauth:2.0:oob'

  # skip_authorization do |resource_owner, client|
  #   client.superapp? or resource_owner.admin?
  # end

  # realm "Doorkeeper"
end

当我访问/oauth/applications时，我得到一个错误页面，上面写着“未初始化的常量Admin”，以及“config/initializers/doorkeeper.rb：18：in 'block（2层）in '"。
我做错了什么？