diff --git a/pkgs/top-level/config.nix b/pkgs/top-level/config.nix
index 8c4438ae154d..a92dd200ccf6 100644
--- a/pkgs/top-level/config.nix
+++ b/pkgs/top-level/config.nix
@@ -9,7 +9,7 @@
 {
   config,
   lib,
-  docPrefix ? "",
+  docPrefix,
   ...
 }:
 
@@ -515,6 +515,7 @@ in
   inherit options;
 
   config = {
+    _module.args.docPrefix = lib.mkDefault "";
     warnings =
       optionals config.warnUndeclaredOptions (
         mapAttrsToList (k: v: "undeclared Nixpkgs option set: config.${k}") config._undeclared or { }