Unlock bootloader issues KIW-L21C432B330 Marshmallow

Hi,
I upgraded to the Marshmallow rom via the OS upgrade early adopters at the weekend but now I am trying to unlock my bootloader to install CM13.

Unfortunately there is no option in the developer settings to 'enable oem unlock' and when I try to unlock i get:

Code:

---

sudo adb reboot bootloader
sudo fastboot oem unlock 1432461542335923
...
FAILED (status read failed (Value too large for defined data type))
finished. total time: 5.005s

---

After this no further fastboot commands work until I unplug and re-connect to the pc.

Is there anyway to enable the oem unlock setting via adb?
Or do I need to go back to a lolipop rom to unlock? If so can anyone tell me how to revert to a previous version.

Thanks.