diff --git a/nsd.nix b/nsd.nix
index 7413f00..553696b 100644
--- a/nsd.nix
+++ b/nsd.nix
@@ -315,7 +315,7 @@ let
 
       ksk = {
         keyFile = mkOption {
-          type = nullOr types.str;
+          type = types.nullOr types.str;
           description =
             "Location of the zone key-signing key file on the local host.";
           default = null;