diff --git a/git/index/base.py b/git/index/base.py
index 47925ad1c..7f53e614a 100644
--- a/git/index/base.py
+++ b/git/index/base.py
@@ -653,7 +653,7 @@ def _to_relative_path(self, path: PathLike) -> PathLike:
             return path
         if self.repo.bare:
             raise InvalidGitRepositoryError("require non-bare repository")
-        if not str(path).startswith(str(self.repo.working_tree_dir)):
+        if not osp.normpath(str(path)).startswith(osp.normpath(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)