ble_gatt: sd_ble_gap_data_length_update() (request) on connection 0x0 returned unexpected value 0x13.

Question

Working on migrating my application from sdk 12.2 to sdk 14 except for some flash issues most of it seems to be working as before, except I notice this message in jlink:

 0> <info> app: Connected
 0> <error> ble_gatt: sd_ble_gap_data_length_update() (request) on connection 0x0 returned unexpected value 0x13.
 0> <info> app: Data len is set to 0x3D(61)

what does this mean?

Edit: using the BLE NUS peripheral example

It seems it comes from this function in #75 nrf_ble_gatt.c:

/**@brief   Start a data length update request.
 * @details This function is called to request a data length update upon connection.
 *          When the peer requests a data length update, sd_ble_gap_data_length_update()
 *          is called directly in response to the BLE_GAP_EVT_DATA_LENGTH_UPDATE event in
 *          on_data_length_update_evt().
 */
static void data_length_update(uint16_t conn_handle, nrf_ble_gatt_t const * p_gatt)
{
    NRF_LOG_DEBUG("Requesting to update data length to %u on connection 0x%x.",
                  p_gatt->links[conn_handle].data_length_desired, conn_handle);

    ble_gap_data_length_params_t const dlp =
    {
        .max_rx_octets  = p_gatt->links[conn_handle].data_length_desired,
        .max_tx_octets  = p_gatt->links[conn_handle].data_length_desired,
        .max_rx_time_us = BLE_GAP_DATA_LENGTH_AUTO,
        .max_tx_time_us = BLE_GAP_DATA_LENGTH_AUTO,
    };

    ret_code_t err_code = sd_ble_gap_data_length_update(conn_handle, &dlp, NULL);
    if (err_code != NRF_SUCCESS)
    {
        NRF_LOG_ERROR("sd_ble_gap_data_length_update() (request)"
                      " on connection 0x%x returned unexpected value 0x%x.",
                      conn_handle, err_code);
    }
}

Petter Myhre · Accepted Answer

I'm sorry, I overlooked that you were using the unmodified example. This seems like a bug to me. 
 Inside gatt_init(), if you change 
 err_code = nrf_ble_gatt_att_mtu_periph_set(&m_gatt, 64);
 
 to 
 err_code = nrf_ble_gatt_att_mtu_periph_set(&m_gatt, BLE_GATT_ATT_MTU_DEFAULT);
 
 and inside sd_config.h, change 
 NRF_SDH_BLE_GATT_MAX_MTU_SIZE 247
 
 to 
 NRF_SDH_BLE_GATT_MAX_MTU_SIZE 23
 
 Edit 22.01.2018 
 This should be fixed in SDK 15.0.0

<error> ble_gatt: sd_ble_gap_data_length_update() (request) on connection 0x0 returned unexpected value 0x13.

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