Why BluetoothSocket.getMaxReceivePacketSize() and BluetoothSocket.getMaxTransmitPacketSize() return 0?

I develop two apps which communicate via Classic Bluetooth - one as a client and the other as a server. I manage to perform successful connection and I want to optimize my read and write operations using BluetoothSocket methods - getMaxReceivePacketSize() and getMaxTransmitPacketSize() methods, but for some reason they both just return 0. What might be the reason for that? Thanks ahead.