diff --git a/pkgs/by-name/op/openmsx/package.nix b/pkgs/by-name/op/openmsx/package.nix
index 89e0ff2e760d..eb1dd576fe45 100644
--- a/pkgs/by-name/op/openmsx/package.nix
+++ b/pkgs/by-name/op/openmsx/package.nix
@@ -71,7 +71,7 @@ stdenv.mkDerivation (finalAttrs: {
 
   # Many thanks @mthuurne from OpenMSX project for providing support to
   # Nixpkgs! :)
-  TCL_CONFIG = "${tcl}/lib/";
+  env.TCL_CONFIG = "${tcl}/lib/";
 
   meta = {
     homepage = "https://openmsx.org";