Ambiguous parent1 and parent2, when use stash list --> compare with previous version

What steps will reproduce the problem?

1. TortoiseGit --> stash list
2. choose one stash and right click --> compare with previous version

What is the expected output? What do you see instead?

expected output:

I want to choose the parent with description as

index(commit-id1)

working tree(commit-id2)

what I see:

What version of TortoiseGit and Git are you using? On what operating system?

TortoiseGit 2.7.0.0 (C:\Program Files\TortoiseGit\bin)

git version 2.18.0.windows.1 (C:\Program Files\Git\bin; C:\Program Files\Git\mingw64; C:\Program Files\Git\mingw64\etc\gitconfig; C:\ProgramData\Git\config)

OS Microsoft Windows 10.0.16299

Please provide any additional information below. documentation about stash, index and working tree

https://stackoverflow.com/questions/27012878/why-is-a-stash-represented-as-2-commits

https://stackoverflow.com/questions/3689838/whats-the-difference-between-head-working-tree-and-index-in-git