builtins.fetchGit error, not a tree object

1

I’m getting this error from builtins.fetchGit:

nix-repl> builtins.fetchGit {
                        url = /mnt/DATA/src/nixpkgs;
                        rev = "a81842b6994ce710ebad13303fde29438f610e8c";
                      }
fetching Git repository '/mnt/DATA/src/nixpkgs'fatal: not a tree object: a81842b6994ce710ebad13303fde29438f610e8c
error: program 'git' failed with exit code 128

But the revision commit is present in the git repo:

[$] git -C /mnt/DATA/src/nixpkgs ls-tree --name-only a81842b6994ce710ebad13303fde29438f610e8c
.editorconfig
.gitattributes
.github
.gitignore
.version
COPYING
README.md
default.nix
doc
lib
maintainers
nixos
pkgs

What is the issue here?

2

The commit must be reachable from the given ref (default master); see also https://github.com/NixOS/nix/issues/2431.