diff --git a/pkgs/top-level/config.nix b/pkgs/top-level/config.nix
index 0ab29d851636..4fedad00f24b 100644
--- a/pkgs/top-level/config.nix
+++ b/pkgs/top-level/config.nix
@@ -35,6 +35,7 @@ let
 
     warnUndeclaredOptions = mkOption {
       description = "Whether to warn when <literal>config</literal> contains an unrecognized attribute.";
+      type = types.bool;
       default = false;
     };