diff --git a/pkgs/build-support/package/make-package.nix b/pkgs/build-support/package/make-package.nix
index fe8196780c40a..80d6e42f7a5f7 100644
--- a/pkgs/build-support/package/make-package.nix
+++ b/pkgs/build-support/package/make-package.nix
@@ -117,13 +117,21 @@ let
 
   layers.withDeps = f: this: old:
     let
-      spy = lib.setFunctionArgs (args: { inherit args; }) (lib.functionArgs f);
+      fargs = lib.functionArgs f;
+      spy = lib.setFunctionArgs
+        (args: { inherit args; })
+        (lib.mapAttrs
+          (_k: _v:
+            # say we have a default, so that unknown attrs aren't a problem and
+            # they can be defined by a subsequent layer.
+            true)
+          fargs);
       # TODO: make callPackage a parameter?
       values = (callPackage spy { }).args;
       old2 = old // {
         deps = old.deps or {} // values;
       };
-      r = f (lib.mapAttrs (name: value: this.deps.${name}) values);
+      r = f (lib.mapAttrs (name: value: this.deps.${name}) fargs);
       r' = if lib.isList r then lib.composeManyExtensions r else r;
     in
       old2 //