Issues installing services in classic mode on Ubuntu core

[MikeB]

I'm trying to install a Debian package that contains a system service
in Classic mode on Ubuntu core.

* I've added a case to /usr/sbin/polivy-rc.d to return 104 for the service.
* The service file is installed at /lib/systemd/system/foo.service.
* There is a dropin file for the service at /etc/systemd/system/foo.d/foo.conf.

I see the following when I install the package using dpkg.

invoke-rc.d: foo.service doesn't exist but the upstart job does.
Nothing to start or stop until a systemd or init job is present.
Failed to get unit file state for foo.service: No such file or directory
foo.service is a disabled or a static unit, not starting it.

Are there any special restrictions on systemd services running in Classic mode?

Regards, Mike