Notices on language negotiation callback when trying to force a full bootstrap

Group has a sub module -- Group language.

/**
 * Implements hook_language_negotiation_info().
 */
function group_language_language_negotiation_info() {
  $providers = array();

  $providers['group-language'] = array(
    'callbacks' => array('language' => 'group_language_from_group_context'),
    'weight' => -5,
    'name' => t('Group'),
    'description' => t("Follow the Group context's language preference."),
    'file' => drupal_get_path('module', 'group_language') .'/group_language.module',
  );

  return $providers;
}

/**
 * Identify language from Group context.
 *
 * @param $languages
 *   An array of valid language objects.
 *
 * @return
 *   A valid language code on success, FALSE otherwise.
 */
function group_language_from_group_context($languages) {
  // We need a full bootstrap in order to get the group context.
  drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);

  if ($group = group_context()) {
  // ... Group' own logic.
  }
}

The drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);
causes:

# Notice: Trying to get property of non-object in drupal_lookup_path()  (line 74 of /var/www/d7_dev/includes/path.inc).
# Notice: Trying to get property of non-object in entity_get_info() (line 6359 of /var/www/d7_dev/includes/common.inc).
# Notice: Trying to get property of non-object in entity_get_info() (line 6359 of /var/www/d7_dev/includes/common.inc).
# Notice: Trying to get property of non-object in locale_init() (line 267 of /var/www/d7_dev/modules/locale/locale.module).
# Notice: Trying to get property of non-object in admin_menu_init() (line 143 of /var/www/d7_dev/sites/all/modules/admin_menu/admin_menu.module).
# Notice: Trying to get property of non-object in entity_get_info() (line 6359 of /var/www/d7_dev/includes/common.inc)

I assume the problem seems to be that the $language object should to be determined, by my negotation callback hasn't returned anything yet, so $language isn't set yet.