diff --git a/modules/clerie-system-upgrade/default.nix b/modules/clerie-system-upgrade/default.nix
index b6c92ba..38eb1c1 100644
--- a/modules/clerie-system-upgrade/default.nix
+++ b/modules/clerie-system-upgrade/default.nix
@@ -21,7 +21,7 @@ in
         description = "Automatically check and install upgrades";
       };
       startAt = mkOption {
-        type = with types; nullOr string;
+        type = with types; nullOr str;
         default = null;
         description = "Systemd time string for starting the unit";
       };