Error adding tunnel to mgre interface

Hi,

I am trying to add a tunnel to an mgre interface.
I can't get past the following error.

$doas ifconfig mgre0 destroy
$doas ifconfig mgre0 create
$doas ifconfig mgre0 tunnel 192.0.2.1 192.0.2.1
ifconfig: SIOCSLIFPHYADDR: Invalid argument
$
$ifconfig mgre
mgre0: flags=8800<SIMPLEX,MULTICAST> mtu 1476
index 9 priority 0 llprio 3
encap: vnetid none
groups: mgre
tunnel: (unset) ttl 64 nodf
$

This is on a default install with patches up to and including 014.
OpenBSD 6.3 GENERIC.MP#7 amd64

If anyone has had success setting up an mgre interface then I would
appreciate a little advice.

Regards

Joe