Bootloader won't lock

I want to re-lock my 6P's bootloader before using it for a trade-in via Project Fi. But every time I try "fastboot flashing lock," a help menu of fastboot commands is all that comes up. For the hell of it, I tried "fastboot oem lock" and "fastboot oem relock," but those just FAIL.

Any ideas? "fastboot devices" works as it should, so it's not like I'm forgetting USB debugging or messing up the USB configuration.