Manually flashed 6.0 stock images, but only get dead android for recovery

I have searched around but cannot find a solution. I can't seem to get a working recovery after flashing stock images with and without TWRP 2.8.7.1.

I followed these steps to upgrade to Android 6.0 after verifying the md5 checksum of my download:
adb reboot bootloader
fastboot flash bootloader bootloader-hammerhead....img [it looks the same as before, but I did it anyway]
reboot bootloader
fastboot flash radio radio-hammerhead....img
reboot bootloader
fastboot flash boot boot.img
fastboot flash recovery recovery.img
fastboot flash system system.img
....I have also tried with and without flashing cache
fastboot reboot
...then I go back into the bootloader and try to access recovery, but I only get the dead android with the red triangle.

Is anyone else having this problem? Does anyone have a suggestion for a fix?
I'm having the same problem with my Nexus 7 (2013).