diff --git a/overlay.nix b/overlay.nix
index ec1dd8f..ad3667e 100644
--- a/overlay.nix
+++ b/overlay.nix
@@ -1,4 +1,6 @@
 final: prev: {
-  formats.xml = import ./xml.nix { pkgs = final; };
+  formats = prev.formats // {
+    xml = import ./xml.nix { pkgs = final; };
+  };
   pythonPackagesExtensions = prev.pythonPackagesExtensions ++ [ (import ./python_overlay.nix) ];
 }