"Enable nested virtualization" is grayed out on the newest MBP Retina

Hello,

I am trying to enable nested virtualization support for Windows Phone 8 emulator in Windows 8.1, but Options -> Optimization tab shows "Enable nested virtualization" grayed, leaving me no chance to turn it on. I know there are certain requirements to the hardware and VM, but this is a brand new MacBook Pro Retina with 16 GB RAM. The VM setup shows 4 CPUs and 8GB, so it's a powerful VM.

Any idea why nested virtualization may be disabled?

Best regards
Vagif

 

This was easy: I just needed to shutdown the VM and the option became enabled.