diff --git a/modules/ppp/default.nix b/modules/ppp/default.nix
index 495cde2..8cef94f 100644
--- a/modules/ppp/default.nix
+++ b/modules/ppp/default.nix
@@ -13,7 +13,11 @@ let
   inherit (lib) mkOption types;
   inherit (pkgs) liminix;
   mkStringOption =
-    description: mkOption { type = types.str; inherit description; };
+    description: mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      inherit description;
+    };
 in {
   options = {
     system.service.pppoe = mkOption {