Mine says the same thing, -1mV. My GPU is idle at the moment, so maybe this changes when something is happening, running a game, etc. Don't really know what it is supposed to be reading myself.
If you are using Avast you can try to temporarily disable Avast 'self protection' (in troubleshooting), run regedit.exe as admin and set Start=4 in HKEY_LOCAL_MACHINE\System\CurrentControlSet\services\aswvmm.
