[WARN] ignoring error reading parent snapshot: TreeStackEmptyError

[vt-alt]

In 0.9.5 I noticed such warning on my tests:

rustic (sisyphus)$ rustic backup .
...
[00:00:00] reading index...               ████████████████████████████████████████          3/3
[00:00:00] getting latest snapshot...     ████████████████████████████████████████          3/3
[INFO] using parent a9f756c6
[INFO] starting to backup . ...
[00:00:00] backing up...                  ████████████████████████████████████░░░░ 450.80 MiB/489.44 MiB 834.63 MiB/s (ETA 0s)
[WARN] ignoring error reading parent snapshot: TreeStackEmptyError
[00:00:03] backing up...                  ████████████████████████████████████████ 489.44 MiB/489.44 MiB 155.93 MiB/s (ETA -)
Files:       0 new, 0 changed, 31508 unchanged
Dirs:        0 new, 0 changed, 5552 unchanged
Added to the repo: 0 B (raw: 0 B)
processed 31508 files, 489.4 MiB
snapshot 96d2a461 successfully saved.
[INFO] backup of . done.

This happens on old and new repositories. The error warnings usually isn't what you want to see in backup software, especially I did nothing suspicious to cause this.