diff --git a/rwx/grub/__init__.py b/rwx/grub/__init__.py
index 3a70ad4..7690a49 100644
--- a/rwx/grub/__init__.py
+++ b/rwx/grub/__init__.py
@@ -17,7 +17,7 @@ MODULES = {
 
 
 def make_image(image_format: str, image_path: str, modules: list[str],
-               memdisk_path: str, pubkey_path: str = None) -> None:
+               memdisk_path: str, pubkey_path: str | None = None) -> None:
     args = [
         ("grub-mkimage",),
         ("--compress", COMPRESSION),