NoMethodError: undefined method `text_field' for nil:NilClass

#!/usr/bin/ruby
$LOAD_PATH << '.'
require 'features/credentials/uat_login'

class Login
    include PageObject
    include PageObject::PageFactory

    def initialize
        username = @browser.text_field(name:"ctl00$ContentPlaceHolder1$Login1$UserName")
        password = @browser.text_field(name:"ctl00$ContentPlaceHolder1$Login1$Password")
        login = @browser.button(name:"ctl00$ContentPlaceHolder1$Login1$loginButton")
    end

    def verifyIsLoginPage()
        @browser.h1.should exist
    end

    def credentials()
        username.send_keys $username
        password.send_keys $password
    end

    def login()
        login.click
    end
end