GELI without passphrase on ZFS root

[Michael ..]

Hi,
 
Has anyone been able to achieve this?
 
I installed FreeBSD 11.2 using AutoZFS option with encryption turned on.  Passphrase is specified as part of install.
 
I want to switch to only a keyfile and no passphrase:
 
geli setkey -K /boot/encryption.key -P /dev/xyz
 
This completes, but I'm still prompted for passphrase on boot.  Nothing appears accepted by the prompt (as the userkey is using only keyfile now?)
 
Setting geom_eli_passphrase_prompt="NO" doesn't help.
 
Michael.