BackgroundProcess::queue() does not return a value

Functions background_process_queue_locked and background_process_queue expect that the BackgroundProcess::queue() method will return a handle or at least non null and non false value. But when you look at that method it does not return a value at all if there was no error:

public function queue($callback, $args = array()) {
    if (!$this->lock(BACKGROUND_PROCESS_STATUS_QUEUED)) {
      return FALSE;
    }

    $this->callback = $callback;
    $this->args = $args;

    if (!background_process_set_process($this->handle, $this->callback, $this->uid, $this->args, $this->token)) {
      // Could not update process
      return NULL;
    }

    module_invoke_all('background_process_pre_execute', $this->handle, $this->callback, $this->args, $this->token);

    // Initialize progress stats
    $old_db = db_set_active('background_process');
    progress_remove_progress($this->handle);
    db_set_active($old_db);

    $queues = variable_get('background_process_queues', array());
    $queue_name = isset($queues[$this->callback]) ? 'bgp:' . $queues[$this->callback] : 'background_process';
    $queue = DrupalQueue::get($queue_name);
    $queue->createItem(array(rawurlencode($this->handle), rawurlencode($this->token)));
    _background_process_ensure_cleanup($this->handle, TRUE);

    // It should return value here!
  }