You are having problems while retrieving results after performing port scanning during internal testing.

The null scan turns off all flags, creating a lack of TCP flags that should never occur in the real world. If the port is closed, a RST frame should be returned and a null scan to an open port results in no response. Unfortunately Microsoft (like usual) decided to completely ignore the standard and do things their own way. Thus this scan type will not work against systems running Windows as they choose not to response at all. This is a good way to distinguish that the system being scanned is running Microsoft Windows.