Why does assert_not flash.empty? not work in integration rails test?

HEre is my code for my integration test:

require 'test_helper'

class UserSignupTest < ActionDispatch::IntegrationTest

  test "a user can signup and return back to root url" do
    visit new_user_path

    fill_in "user[username]", with: "flyers1"
    fill_in "user[first_name]", with: "user"
    fill_in "user[last_name]", with: "name"
    fill_in "user[email]", with: "username@gmail.com"
    fill_in "user[street]", with: "street"
    fill_in "user[city]", with: "city"
    fill_in "user[state]", with: "state"
    fill_in "user[country]", with: "country"
    fill_in "user[password]", with: "password"
    fill_in "user[password_confirmation]", with: "password"
    click_button "Finish Signup"

    refute flash.empty?
end

Here is my controller method for create:

def create
    user = User.new(user_params)
    if user.valid?
      valid_user_signup(user)
    else
      invalid_user_signup(user)
    end
  end

and

def valid_user_signup(user)
    user.save
    session[:user_id] = user.id
    flash[:success] = "Signup Successful"
    redirect_to root_path
  end

What is going on here? Why do I get an error message that says "undefined method flash" for nil:nilclass?