[Reg 5.15.2 -> 5.15.9] QModbusClient:: sendRawRequest() produces ModbusDevice::UnknownError when there is no error

auto client = new QModbusRtuSerialMaster;
client->connectDevice();

quint16 subfunction = 0;
quint16 data = 0xA537;
auto reply = client->sendRawRequest(QModbusRequest(QModbusRequest::Diagnostics, subfunction, data), 1);
QObject::connect(reply, &QModbusReply::finished, [=]
{
    qDebug("Response received");
    qDebug() << "\tReply error:" << reply->error();
    qDebug() << "\tClient error:" << client->error();
    {
        quint16 replySubfunction;
        quint16 replyData;
        QModbusResponse response = reply->rawResult();
        response.decodeData(&replySubfunction, &replyData);

        qDebug() << "\tRaw response:" << response;
        qDebug() << "\tFunction:" << response.functionCode() << "\tSub-function:" << replySubfunction << "\tData:" << QByteArray::number(replyData, 16).prepend("0x");
    }

    reply->deleteLater();
});

Client connected
Response received
    Reply error: QModbusDevice::NoError
    Client error: QModbusDevice::NoError
    Raw response: 0x080000a537
    Function: 8     Sub-function: 0     Data: "0xa537"

Client connected
Response received
    Reply error: QModbusDevice::UnknownError
    Client error: QModbusDevice::NoError
    Raw response: 0x080000a537
    Function: 8     Sub-function: 0     Data: "0xa537"