Tunneling GRE over GUE over IPv6

Hello,

I would like to set up a GRE tunnel using GUE or FOU, but it doesn't
seem to be working:

root@host1:~# ip link add ip6greguetunnel type ip6gre remote fd6c:ffa:eb7a:74e2::141 local fd6c:ffa:eb7a:74e2::171 encap gue encap-dport 6661
RTNETLINK answers: Invalid argument
root@host1:~# ip link add ip6greguetunnel type ip6gre remote fd6c:ffa:eb7a:74e2::141 local fd6c:ffa:eb7a:74e2::171 encap fou encap-dport 6661
RTNETLINK answers: Invalid argument

Does anyone know why? What am I doing wrong? What does "Invalid
argument" mean in this context?

If this isn't the right place to ask, I would appreciate being directed
to the right place :-)

Best regards,
Marek Küthe

-- 
Marek Küthe
m.k@mk16.de
er/ihm he/him