KeyError 'system' while executing tetherback

While executing tetherback i get the following error:
```
tetherback v0.9.1
Found ADB version 1.0.40
Using default transfer method: adb exec-out pipe (--exec-out)
Device reports kernel 3.18.71-perf+
Device reports TWRP version 3.2.3
Reading partition map for mmcblk0 (2 partitions)...
partition map: 100%
Traceback (most recent call last):
File "/home/byte/Documents/phone/nand/env/bin/tetherback", line 10, in
sys.exit(main())
File "/home/byte/Documents/phone/nand/env/lib/python3.7/site-packages/tetherback/tetherback.py", line 338, in main
plan = plan_backup(args, partmap)
File "/home/byte/Documents/phone/nand/env/lib/python3.7/site-packages/tetherback/tetherback.py", line 206, in plan_backup
plan.update((p,BackupPlan('%s.%s.win'%(p, partmap[p].fstype), '-p')) for p in mp)
File "/home/byte/Documents/phone/nand/env/lib/python3.7/site-packages/tetherback/tetherback.py", line 206, in
plan.update((p,BackupPlan('%s.%s.win'%(p, partmap[p].fstype), '-p')) for p in mp)
KeyError: 'system'
```
The phone is in recovery and recognized by ADB, so i dont know what could be causing this.
I tried both the release and latest version.

Host OS: ArchLinux `Linux sigma 5.0.2-arch1-1-ARCH #1 SMP PREEMPT Thu Mar 14 18:47:49 UTC 2019 x86_64 GNU/Linux`
Phone: LG-G6 (Codename: H870) using [HavocOS Unofficial](https://forum.xda-developers.com/lg-g6/development/rom-havocos-v2-1-t3893668)
Python Version: `Python 3.7.2`
Pip Version: `pip 19.0.3 from /home/byte/Documents/phone/nand/env/lib/python3.7/site-packages/pip (python 3.7)`
I am using a virtualenv