Error during Rebase Edit/Split on a root path

Using TortoiseGit 1.8.15.2.

With SUBST used to point S: at the base of a Git checkout (ie. S:\.git exists), performing a rebase normally works as expected.

However, when selecting a revision with Edit, and then ticking "Edit/Split commit", something odd happens.

First, the subsequent Commit window comes up with "S:\S:\ - Commit - TortoiseGit" as its caption. (Note this does not happen in a normal Commit outside of the rebase.)

Second, each time the Commit window reopens, before it displays the list of uncommitted files it displays the following error:

Failed to get UnRev file list

fatal: S:": 'S:"' is outside repository

(It's a little unclear in that but note that the quotes are also out of sequence: S:": 'S:"'.)

After acknowledging the error, the list of uncommitted files comes up as expected, and the actual split and the overall rebase still works correctly.