Central for vendor-specific services shows characteristic uuids = 0

Question

Hi,

I am developing a system consisting of a BLE peripheral and central with a custom service (SDK 17, S140, PCA10056).

For the peripheral, I am using a modified version from the Nordic Playground Bluetooth Course. I upgraded it to work in SDK 17 and I tested it using nRF Connect, where I have no issues reading the characteristic, enabling notifications and writing to it after bonding using LESC and MITM OFF settings.

For the central, I started from the HRS example, and now I have both devices bonding and transferring data using a notification, but I am confused with some changes I had to make to the central to make it work:

The custom service on the peripheral uses BLE_UUID_TYPE_VENDOR_BEGIN and a 128-bit uuid. However, I must initialize the central service to look for a BLE_UUID_TYPE_BLE; if I select BLE_UUID_TYPE_VENDOR_BEGIN instead, I get error messages:
<error> nrf_ble_gq: SD GATT procedure (2) failed on connection handle 0 with error: 0x00000007.
<debug> ble_cus_c: Battery Service discovery failure at peer.

And in ble_cus_on_db_disc_evt(), I get p_evt->evt_type = BLE_DB_DISCOVERY_ERROR, although instantly after I get BLE_DB_DISCOVERY_AVAILABLE. I never get the complete event after this though, which I do if I don't use vendor-specific.

Should I keep using BLE_UUID_TYPE_BLE in this case? Why does it work for a vendor-specific ID?

uint32_t ble_cus_c_init(ble_cus_c_t * p_ble_cus_c, ble_cus_c_init_t * p_ble_cus_c_init)
{
    VERIFY_PARAM_NOT_NULL(p_ble_cus_c);
    VERIFY_PARAM_NOT_NULL(p_ble_cus_c_init);

    ble_uuid_t cus_uuid;

    cus_uuid.type                = BLE_UUID_TYPE_BLE;
    cus_uuid.uuid                = BLE_UUID_CUSTOM_SERVICE;

    p_ble_cus_c->conn_handle                = BLE_CONN_HANDLE_INVALID;
    p_ble_cus_c->peer_cus_db.bl_cccd_handle = BLE_GATT_HANDLE_INVALID;
    p_ble_cus_c->peer_cus_db.bl_handle      = BLE_GATT_HANDLE_INVALID;
    p_ble_cus_c->evt_handler                = p_ble_cus_c_init->evt_handler;
    p_ble_cus_c->error_handler              = p_ble_cus_c_init->error_handler;
    p_ble_cus_c->p_gatt_queue               = p_ble_cus_c_init->p_gatt_queue;

    return ble_db_discovery_evt_register(&cus_uuid);
}

Even if I use the above workaround, I receive a characteristic with uuid=0x0000. Because of this, I can't check if the IDs match. The only way I can proceed at this time is to ignore this error and assume that this is the characteristic I am searching for given that there is one characteristic per service. However, this is not ideal, as I would like to have multiple characteristics per service. Why does this happen?

Please see attached code. Thanks for your help.

central_peripheral.zip