Upgrading to M preflash verification failed

Hi guys.

Got my nexus 6 and decided to update to M

bootloader unlocked.
flashing with nexus root kit 2.0.5
up until the adb is trying to flash the system

it said preflash verification fail.

Then the device boot back to 5.1.1

I have flash back to facotry 5.1.1 now and I had no problem flashing the image of 5.1.1.

Please help