git-subtrac runs into `git-subtrac: git status` errors all the time

I'm having issues getting git-subtrac to work with my repo https://github.com/telotortium/emacs.d, which uses several submodules. After running `git clone https://github.com/telotortium/emacs.d` and `git submodule update --init --recursive` inside the repo, I can run `git subtrac --auto-exclude update`. I then want to follow the advice of the README and set `url = .` for all the submodules in `.gitmodules`. I do this, and then attempt to run `git fetch --all` in all the submodules. When I go back up to the main repo, I then encounter errors like this:

```
git subtrac --verbose --auto-exclude update
Scanning branch: refs/heads/master
add 160b1c4552 refs/heads/master/cache/
Searching submodules for: refs/heads/master/lisp/git-auto-commit-mode@f89ed36c38
git-subtrac: git status(lisp/org-gtasks.git): invalid checksum (fetch it manually? or try --exclude)
```

The specific submodule in the error message changes when I run the command repeatedly.