Testing Devise - Capybara :: ElementNotFound: cannot find xpath "/ html"

I am trying to test login functionality on my site using RSpec, Capybara and Factory Girl. I always get the following error in the terminal, but cannot find any solution. When I use save_and_open_page, I get a blank page. Hope someone knows what happened. Thank!

Here test output

Failure/Error: expect(page).to have_text("Login successful.")
     Capybara::ElementNotFound:
       Unable to find xpath "/html"
     # ./spec/features/navigation_spec.rb:49:in `block (2 levels) in <top (required)>'

Here is my file sometest_spec.rbwith the corresponding lines:

require 'spec_helper'
include Warden::Test::Helpers

describe "User login" do
  it "log in normal user" do

    Warden.test_mode!

    user = create(:normal_user)
    user.confirmed_at = Time.now
    user.save
    login_as(user, scope: :user)

    expect(page).to have_text("Login successful.")

    logout(:user)
    Warden.test_reset!
  end
end

Here is my factory_girl.rbfile

RSpec.configure do |config|
  config.include FactoryGirl::Syntax::Methods
end

And file factories.rb

FactoryGirl.define do
  factory :normal_user, class: User do
    firstname "John"
    lastname "Doe"
    email "test@user.com"
    password "thepassword"
    admin false
  end
end