diff --git a/nix/module.nix b/nix/module.nix
index a5bca41..465bc90 100644
--- a/nix/module.nix
+++ b/nix/module.nix
@@ -38,6 +38,7 @@ in {
     };
     accessKeyFile = mkOption {
       type = types.nullOr types.str;
+      default = null;
       description = "access key file path";
     };