diff --git a/backup.nix b/backup.nix
index f29dea3..7e95b6a 100644
--- a/backup.nix
+++ b/backup.nix
@@ -34,7 +34,7 @@ in
       description = "Schedule for backups. Needs to be valid systemd OnCalendar value.";
     };
     password-file = lib.mkOption {
-      type = lib.types.str;
+      type = lib.types.nullOr lib.types.str;
       default = null;
       description = "File containing the restic password.";
     };