Undefined index: #id в функции nucleus_preprocess_fieldset()

Hello!

I'm using TB Purity theme based on Nucleus (Great Thanks for these splendid themes). But the error appears almost on all pages of my site:

Notice: Undefined index: #id in function nucleus_preprocess_fieldset() (line 365 in /sites/all/themes/nucleus/nucleus/inc/preprocess_functions.inc).

Here is the whole function:

function nucleus_preprocess_fieldset(&$vars) {
  $element = $vars['element'];
  $vars['attributes'] = $element['#attributes'];
  $vars['attributes']['id'][] = $element['#id'];
  $vars['attributes']['class'][] = 'fieldset';
  $vars['attributes']['class'][] = 'form-wrapper';
  if (!empty($element['#title'])) {
    $vars['attributes']['class'][] = 'titled';
  }

The error appears only under anonimous user. Please, can You help me with this error?

Great Thanks!