Why pundit can't find policies in controller tests?

The controller:

class UsersController < ApplicationController
  def index
    ...
    authorize User
  end
  ...

The policy:

class UserPolicy < ApplicationPolicy
  def index
    @user.admin?
  end
end

The test:

class UsersControllerAuthorizationTest < ActionController::TestCase
  tests :users

  def user
    @user ||= create(:user)
  end

  test 'should not authorize ordinary users to access the page' do
    sign_in user
    get :index
    assert_response :error
  end
end

The app fails with Pundit::NotAuthorizedError (not allowed to index? this User) as expected. But the test says:

Pundit::NotDefinedError: unable to find policy UserPolicy for User

Am I doing it wrong? Can I make it find the policy?