diff --git a/lib/fudo/domains.nix b/lib/fudo/domains.nix
index a3f52c0..8ff2a62 100644
--- a/lib/fudo/domains.nix
+++ b/lib/fudo/domains.nix
@@ -120,8 +120,9 @@ let
         };
 
         gssapi-realm = mkOption {
-          type = str;
+          type = nullOr str;
           description = "GSSAPI (i.e. Kerberos) realm of this domain.";
+          default = null;
         };
 
         kerberos-master = mkOption {