How to write integration test for JSON in minitest

I've seen post answering this for RSpec but I'm using minitest for testing. Does anyone know how to properly write the test below? Because currently it generates an error referring to its first test line:

NoMethodError: undefined method `cookies' for nil:NilClass

test "if already logged in" do
  # To set two cookies
  request.cookies["HTTP_EMAIL"] = @user1.email
  request.cookies["Authorization"] = 'mytoken'
  # Trying to log in as different user
  log_in_as(@user2)    # This uses a test helper that I already found works fine
  # We should get back JSON
  response.header['Content-Type'].should include 'application/json'
  assert_response :success
  # It should respond with a specific message
  assert_select "fc", JSON.parse(response.body)["Already logged in"]
  # Cookies should not have changed. Original user still logged in, not a new user
  assert_equal cookies['HTTP_EMAIL'], @orguser_admin.email
  assert_equal cookies['Authorization'], "mytoken"
end