diff --git a/lib/attrsets.nix b/lib/attrsets.nix
index 1e4142562fa..9bac69af34f 100644
--- a/lib/attrsets.nix
+++ b/lib/attrsets.nix
@@ -440,7 +440,7 @@ rec {
       => { a = { b = 6; d = 2; }; }
   */
   overrideExisting = old: new:
-    old // listToAttrs (map (attr: nameValuePair attr (attrByPath [attr] old.${attr} new)) (attrNames old));
+    mapAttrs (name: value: new.${name} or value) old;
 
   /* Get a package output.
      If no output is found, fallback to `.out` and then to the default.