R&S CMU200 requires both primary and secondary addresses

I have been trying to talk to a Rhode and Schwarz CMU200 Communication test set.
After several hours of grief, I found that the device does respond if I address it with both the primary AND secondary addresses but not if I use the PID alone (with ibtest)

$ ibterm -p 22 -s 96

(ibtest doesn't seem to have a way of setting the secondary address.)

Is it normal (allowed) for a GPIB (IEEE 488.2) device to not respond to a command when only addressed with the PID?
I can't find any reference to this in the manual for the CMU200.

Michael