diff --git a/pkgs/development/compilers/mozart/binary.nix b/pkgs/development/compilers/mozart/binary.nix
index a19aac53d5de..382cb2fa6cfd 100644
--- a/pkgs/development/compilers/mozart/binary.nix
+++ b/pkgs/development/compilers/mozart/binary.nix
@@ -39,7 +39,7 @@ stdenv.mkDerivation {
     tk-8_5
   ];
 
-  TK_LIBRARY = "${tk-8_5}/lib/tk8.5";
+  env.TK_LIBRARY = "${tk-8_5}/lib/tk8.5";
 
   nativeBuildInputs = [ makeWrapper ];