I merged 5.2.0 into my .propellor, last merge was merging f6797bed.

Since the merge, when I try to spin, I get:

riva4.ni.fr.eu.org has ipv4 ... ok
** warning: Unknown host OS is not supported by this property.
CallStack (from HasCallStack):
  error, called at src/Propellor/Property.hs:350:30 in main:Propellor.Property
riva4.ni.fr.eu.org container vz-web2 ... failed
riva4.ni.fr.eu.org overall ... failed

I have in my config.hs:

riva4 :: Host   
riva4 = host "riva4.ni.fr.eu.org" $ props
    & ipv4 ""
    & stdContainerSpawn "vz-web2" "2g" vzWeb2

stdContainerSpawn :: Systemd.MachineName
          -> String
          -> Systemd.Container
          -> Property (HasInfo + DebianLike)
stdContainerSpawn name size container =
    Lvm.lvFormatted Lvm.YesReallyFormatLogicalVolume
        (Lvm.LogicalVolume name (Lvm.VolumeGroup "vg0")) size
        `before` Fstab.mounted "auto" dev dir mempty
        `before` Systemd.nspawned container
        `describe` ("container " ++ name)
    dev = "/dev/vg0" </> name
    dir = "/var/lib/container" </> name

vzWeb2 :: Systemd.Container
vzWeb2 = Systemd.debContainer "vz-web2" $ props
    & osDebian (Stable "stretch") X86_64
    & ipv4 ""

I reviewed all changes in propellor, but I cannot find what can cause this.

How can I debug this?