Cannot validate settings form

Cannot submit settings form, because $values['dlayout_max_width']['#default_value'] doesn't exist, only $values['dlayout_max_width']

/**
 * Validate the form.
 */
function storefront_theme_settings_validate($form, &$form_state) {

  $values = $form_state['values'];

  // Validate max_width values seperatly.
  if ($values['dlayout_set_max_width'] == 1) {
    if (empty($values['dlayout_max_width']['#default_value'])) {
      form_set_error('dlayout_max_width', t('Standard layout max-width is empty - you forgot to enter a value for the max width!'));
    }
  }
}