diff --git a/lib/helpers.nix b/lib/helpers.nix
index 51c867a3..7e8b44a0 100644
--- a/lib/helpers.nix
+++ b/lib/helpers.nix
@@ -89,7 +89,7 @@ rec {
 
     mkInt = default: mkNullable lib.types.int (toString default);
     mkBool = default: mkNullable lib.types.bool (if default then "true" else "false");
-    mkStr = default: mkNullable lib.types.str ''"${default}"'';
+    mkStr = default: mkNullable lib.types.str ''${builtins.toString default}'';
     mkEnum = enum: default: mkNullable (lib.types.enum enum) ''"${default}"'';
     mkEnumFirstDefault = enum: mkEnum enum (head enum);
   };