Cannot add-unit for etcd, "series does not match"

I’m trying to get a healthy etcd cluster, whilst upgrading my nodes to ubuntu 20.04, coming from 18.04.

When trying to add a new etcd, I get this:

juju add-unit etcd --to 94
ERROR acquiring machine to host unit "etcd/45": cannot assign unit "etcd/45" to machine 94: series does not match

Well, this is new to me, applications can have a series as well.

Surely the error can be made more user friendly? E.g.

cannot assign unit "etcd/45" to machine 94: series of application (bionic) is incompatible with the machine's series (focal). Set the application series with `juju set-series` or provide a machine based on the focal series."

Applications do have a series (or base). It’s used to determine what type of machine to create when a unit is added via juju add-unit.

juju upgrade-machine assists in upgrading an existing juju machine with units.

juju set-application-base assists in changing the base (or series) used for future units of that application.