Comment 7 for bug 1908427

> I guess. ie, choose a default platform when we don't have one.

This should always be amd64.

> One other thing: could you perhaps show the exact payload/request you're sending to refresh to check whether it's the same platform issue or maybe a third different problem?

I dug this out from the logs, which is to say:

machine-0: 09:29:05 DEBUG juju.apiserver.charms Locate charm using: Refresh one (instanceKey: 67cf7ef4-37bf-401e-853d-5ea34dec28b6): using ID NKescbK9jZs3Zvs1AsIU96lRPz666BN5 revision 2, with channel latest/stable and platform s390x/ubuntu/focal