KeyError: 'system', with TWRP-style backup

I have a LLD-L31 (`ieland`) with [TWRP 3.6.1_9-0 for `anne`](https://eu.dl.twrp.me/anne/) and `tetherback -0vv` returns:
```tetherback v0.9.1
ADB: ('adb', '-d', 'version')
Found ADB version 1.0.41
Using default transfer method: adb exec-out pipe (--exec-out)
ADB: ('adb', '-d', 'shell', 'uname -r')
Device reports kernel 4.9.148
ADB: ('adb', '-d', 'shell', 'twrp -v')
Device reports TWRP version 3.6.1_9-0
ADB: ('adb', '-d', 'shell', 'cat /etc/fstab')
ADB: ('adb', '-d', 'shell', 'cd /sys/block; ls -d mmcblk*')
ADB: ('adb', '-d', 'shell', 'cat "/sys/block/mmcblk0/uevent"')
Reading partition map for mmcblk0 (0 partitions)...
partition map: 100%
ADB: ('adb', '-d', 'shell', 'cat "/sys/block/mmcblk0boot0/uevent"')
Reading partition map for mmcblk0boot0 (0 partitions)...
partition map: 100%
ADB: ('adb', '-d', 'shell', 'cat "/sys/block/mmcblk0boot1/uevent"')
Reading partition map for mmcblk0boot1 (0 partitions)...
partition map: 100%
ADB: ('adb', '-d', 'shell', 'cat "/sys/block/mmcblk0rpmb/uevent"')
Reading partition map for mmcblk0rpmb (0 partitions)...
partition map: 100%
ADB: ('adb', '-d', 'shell', 'cat "/sys/block/mmcblk1/uevent"')
Reading partition map for mmcblk1 (0 partitions)...
partition map: 100%
Traceback (most recent call last):
File "/usr/bin/tetherback", line 33, in
sys.exit(load_entry_point('tetherback==0.9.1', 'console_scripts', 'tetherback')())
File "/usr/lib/python3.10/site-packages/tetherback/tetherback.py", line 317, in main
plan = plan_backup(args, partmap)
File "/usr/lib/python3.10/site-packages/tetherback/tetherback.py", line 190, in plan_backup
plan.update((p,BackupPlan('%s.%s.win'%(p, partmap[p].fstype), '-p')) for p in mp)
File "/usr/lib/python3.10/site-packages/tetherback/tetherback.py", line 190, in
plan.update((p,BackupPlan('%s.%s.win'%(p, partmap[p].fstype), '-p')) for p in mp)
KeyError: 'system'
```
I also tried with [TWRP 3.2.1 Pretorian](https://forum.xda-developers.com/t/recovery-emui-8-x-honor-9-lite-unofficial-twrp-3-2-1-0-12-05-2018.3789306/) but it encounters the same error