diff --git a/pkgs/by-name/qu/qucs-s/package.nix b/pkgs/by-name/qu/qucs-s/package.nix
index 2bcc4611adf1..c53546c3ee8a 100644
--- a/pkgs/by-name/qu/qucs-s/package.nix
+++ b/pkgs/by-name/qu/qucs-s/package.nix
@@ -83,7 +83,7 @@ stdenv.mkDerivation (finalAttrs: {
     (lib.makeBinPath kernels)
   ];
 
-  QTDIR = qt6Packages.qtbase.dev;
+  env.QTDIR = qt6Packages.qtbase.dev;
 
   doInstallCheck = true;
   installCheck = ''