Undefined Variable Help

[wyclef]

Hi, I am getting an undefined variable error with $page wondering how I can adjust this to account for that?

<?php
  if (get_field('external-site', $page->ID) == '') {
    wp_redirect( get_field('external-site', $page->ID) );
  } else {
    wp_redirect( home_url().'/page-not-found' );
  }
?>