WPScan shows PHP Notice: Undefined index: HTTP_HOST in debug log file

Hi there,

I currently have WPScan installed on a clients website and the report shows that the debug log file is showing a PHP notice when I open it.

PHP Notice: Undefined index: HTTP_HOST in /www/public_html/wp-content/plugins/headers-security-advanced-hsts-wp/headers-security-advanced-hsts-wp.php on line 131

This is the section of code where this error is flagged:

}
add_filter( 'wp_headers', 'Headers_Security_Advanced_HSTS_WP_Headers' );

// check HTTPS - preload list HSTS
if (!isset($_SERVER['HTTPS']) || $_SERVER['HTTPS'] !== 'on') { 
  if(!headers_sent()) {
      header("Status: 301 Moved Permanently");
      header(sprintf(
          'Location: https://%s%s',
          $_SERVER['HTTP_HOST'],
          $_SERVER['REQUEST_URI']
      ));
      exit();
  }

I am not a coder or developer so unsure if this is something to worry about. Any advice or insight would be greatly appreciated.

Thanks

Paul