String variable shows as empty but length shows it to not be

String variable shows as empty but length shows it to not be

I'm using VB.net Visual Studio 2008 for a test fixture, writing and
reading 4 USB virtual serial ports using the serialport class.
The odd thing that's happening is that, looking at the code below,
ReceivedString shows, on mouseover, to be "" or empty yet
ReceivedStringLength says 37. How is this possible? Both variables are
declared locally in a private sub. Has anyone encountered this situation
and could it be a bug in Visual Studio 2008?
ReceivedString = SerialPortMaestro.ReadExisting
ReceivedStringLength = Len(ReceivedString)