Failed to addClass() in page preprocess

I've implemented Preprocess plugin for page and trying to run this code in proprocessVariables() method:

if (array_key_exists('newsmenu', $variables['page']['header'])) {
      $variables->addClass('with-section-menu', DrupalAttributes::CONTENT);
}

but this code fails with

Warning: array_unique() expects parameter 1 to be array, object given in Drupal\bootstrap\Utility\Attributes->getClasses() (line 63 of themes/bootstrap/src/Utility/Attributes.php).

Here is method code

public function &getClasses() {
    $classes = &$this->offsetGet('class', []);
    $classes = array_unique($classes);
    return $classes;
  }

and in case there is no defined classes yet $this->offsetGet('class', []) returns instance of \Drupal\Core\Template\AttributeArray instead of just array.

Is this a bug or I'm doing something wrong?