"What is a State Machine?" Lesson user_notifier error

test: #send_request_email should send an email. (UserFriendshipTest):
NoMethodError: undefined method `friend' for UserNotifier:Class

context "#send_request_email" do
  setup do
    @user_friendship = UserFriendship.create user: users(:allan), friend: users(:mike)
  end

  should "send an email" do
    assert_difference 'ActionMailer::Base.deliveries.size', 1 do
      @user_friendship.send_request_email
    end
  end
end

user_notifier.rb :

class UserNotifier < ActionMailer::Base
  default from: "from@example.com"

  def friend_requested(user_friendship_id)
    user_friendship = UserFriendship.find(user_friendship_id)

    @user = user_friendship.user
    @friend = user_friendship.friend

    mail to: @friend.email,
        subject: "#{@user.first_name} wants to be friends on Treebook"
    end
end