QIODevice calls readData twice for an unbuffered open mode

I try to port an application from qt 4.8 to 5.0. And the implementation of QIODevice::read(char *data, qint64 maxSize) has changed. I call read with maxSize = 1 to my IODevice (read/write/unbuffered) and in my subclass readData is called twice. First call is ok (with readData(data, 1)), second call is called with readData(data, 0). caused by line 923 of qiodevice.cpp. With the second call my IODevice (Pipe under Windows hangs).

Is the second call for a unbuffered IODevice a bug or feature?