diff --git a/git/index/base.py b/git/index/base.py
@@ -655,7 +655,10 @@ def _to_relative_path(self, path: PathLike) -> PathLike:
            raise InvalidGitRepositoryError("require non-bare repository")
        if not osp.normpath(str(path)).startswith(str(self.repo.working_tree_dir)):
            raise ValueError("Absolute path %r is not in git repository at %r" % (path, self.repo.working_tree_dir))
-        return os.path.relpath(path, self.repo.working_tree_dir)

    def _preprocess_add_items(
        self, items: Union[PathLike, Sequence[Union[PathLike, Blob, BaseIndexEntry, "Submodule"]]]

diff --git a/test/test_index.py b/test/test_index.py
@@ -16,6 +16,7 @@
import subprocess
import sys
import tempfile

from gitdb.base import IStream

@@ -1015,6 +1016,27 @@ class Mocked:
        rel = index._to_relative_path(path)
        self.assertEqual(rel, os.path.relpath(path, root))

    @pytest.mark.xfail(
        type(_win_bash_status) is WinBashStatus.Absent,
        reason="Can't run a hook on Windows without bash.exe.",