My bluetooth connexion is reading only zeros

From my Android phone, I'm trying to read (using Bluetooth) incomming strings from an external GPS device. I've followed mainly the BluetoothChat example and everything seems to work as expected so far. My reading thread is executing and I can see variable bytes packets incoming when looping with the following code:

Log.d(TAG, "BEGIN mConnectedThread");

byte[] buffer = new byte[1024];

int bytes;

// Keep listening to the InputStream while connected

while (true) 

{

    try 

    {                   

        bytes = mmInStream.read(buffer);



        // Test...

        String strReadBuf = new String(buffer, 0, bytes);   



       // Send the obtained bytes to the UI Activity

       mHandler.obtainMessage(BluetoothHandler.MessageType.READ,

            bytes, buffer).sendToTarget();

    } catch (IOException e) {

         Log.e(TAG, "disconnected", e);

         sendErrorMessage(R.string.bt_connection_lost);

         break;

    }

 }

The strings I'm supposed to read are text strings (NMEA format) but I'm reading only 0 and -32 bytes in my buffer array. Any idea why I'm getting this?