Audio only outputs not working via orchestration UI

[voltagectrl]

STORY:
As a user I want to use audio only outputs in my production to enable the use of mixminus return feeds without having to waste bandwith on a full video return feed

INFO
Using the attached mixerconfig json
mixerconfig-iso-pfl-mixminus.json

ISSUE
Trying to start a prodution where a stream is attached to the mixminus output in above mixerconfig failes, outputting the following error: