IPC exception: Policy append: rule: Invalid parent ID despite no parent-hash in rules.conf

[jean-christophe-manciot]

Ubuntu impish usbguard 1.0.0+ds-2

# systemctl status usbguard
● usbguard.service - USBGuard daemon
     Loaded: loaded (/lib/systemd/system/usbguard.service; enabled; vendor preset: enabled)
     Active: active (running) since Mon 2021-07-19 20:10:39 CEST; 15h ago
       Docs: man:usbguard-daemon(8)
   Main PID: 1326 (usbguard-daemon)
      Tasks: 3 (limit: 19016)
     Memory: 5.8M
     CGroup: /system.slice/usbguard.service
             └─1326 /usr/sbin/usbguard-daemon -f -s -c /etc/usbguard/usbguard-daemon.conf

...
Jul 19 20:10:39 hostname usbguard-daemon[1326]: uid=0 pid=1262 result='SUCCESS' device.system_name='/devices/pci0000:00/0000:00:14.0/usb4/4-2/4-2.4' t>
Jul 19 20:11:23 hostname usbguard-daemon[1326]: IPC: client_pid=1259: IPC exception: Policy append: rule: Invalid parent ID

# systemctl status usbguard-dbus
● usbguard-dbus.service - USBGuard D-Bus Service
     Loaded: loaded (/lib/systemd/system/usbguard-dbus.service; enabled; vendor preset: enabled)
     Active: active (running) since Mon 2021-07-19 20:10:39 CEST; 16h ago
       Docs: man:usbguard-dbus(8)
   Main PID: 1259 (usbguard-dbus)
      Tasks: 4 (limit: 19016)
     Memory: 5.2M
     CGroup: /system.slice/usbguard-dbus.service
             └─1259 /usr/sbin/usbguard-dbus --system

Jul 19 20:10:39 hostname systemd[1]: Starting USBGuard D-Bus Service...
Jul 19 20:10:39 hostname systemd[1]: Started USBGuard D-Bus Service.

Despite:

# grep parent /etc/usbguard/rules.conf
#