`mstro_subscription_poll` does not work for `MSTRO_POOL_EVENT_OFFER`

I would like to subscribe to the offer event. Here is my code :

  mstro_subscription offer_subscription;
  mstro_subscribe(NULL, MSTRO_POOL_EVENT_OFFER, MSTRO_SUBSCRIPTION_OPTS_DEFAULT,
                  &offer_subscription);
  mstro_pool_event ev;
  while (true)
  {
    mstro_subscription_poll(offer_subscription, &ev);
    while (ev != NULL)
    {
      switch (ev->kind)
      {
      case MSTRO_POOL_EVENT_OFFER:
        debug("CDO %s offered", ev->offer.cdo_name);
        break;
      default:
        debug("Unexpected event %d\n", ev->kind);
        break;
      }
    }
  }

Multiple CDOs had been offered however ev is always NULL.