How do an HID reply to IN get report when nothing to say ?

[CAUCHARD]

I use ST µPSD3254A for USB communication.
I use HID reports to exchange datas.
Their (ST) application note says something like: "when nothing to send to host, the endpoint should send ZLP (zero lenght packet) the IN get report request from Host".
I have seen in the "USB complete" book that NACK in this case should be more appropriate as an answer.
What is your opinion? is the fact I answer ZLP a problem for Host USB controler ?
Thanks in advance !

[Jan Axelson]

On the interrupt endpoint, return NAK.

In response to a Get_Report control request, return a ZLP in the data stage.

Jan