diff --git a/git-auto-commit-mode.el b/git-auto-commit-mode.el
index eaeda3a..c027d5f 100644
--- a/git-auto-commit-mode.el
+++ b/git-auto-commit-mode.el
@@ -118,8 +118,9 @@ It can be:
                  (function :tag "Function")))
 
 (defun gac-relative-file-name (filename)
-  "Find the path to FILENAME relative to the git directory."
-  (let* ((git-dir
+  "Find the path to FILENAME relative to its git directory."
+  (let* ((default-directory (file-name-directory (expand-file-name filename)))
+         (git-dir
           (string-trim-right
            (shell-command-to-string "git rev-parse --show-toplevel"))))
     (file-relative-name filename git-dir)))