Cisco IOS XR, how to configure SPAN to mirror traffic from one sub-interface to another sub-interface?

Title says it all. When I configure the following:

monitor-session M1 ethernet destination interface HundredGigE0/0/0/0.40 exit interface HundredGigE0/0/0/0.20 ipv4 address 10.20.20.1 255.255.255.0 ipv6 address fdd1:10:20:20::1/64 encapsulation dot1q 20 monitor-session M1 ethernet 

And try to commit, I get this error:

interface HundredGigE0/0/0/0.20 monitor-session M1 ethernet !!% 'SPAN' detected the 'warning' condition 'The platform encountered an error processing a SPAN operation': Operation not supported 

I tried changing the interface monitor-session command to include direction, but nothing works. SPAN works when configuring physical interface to physical interface.

Note: adding "port-level" to the end of the monitor-session M1 ethernet on the interface does not take (not recognized). Also running version 7.4.15.

How does one configure mirroring of one sub interface to another sub interface?