diff --git a/pkgs/by-name/gt/gtypist/package.nix b/pkgs/by-name/gt/gtypist/package.nix
index facac531cc67..5b3c20feb5b0 100644
--- a/pkgs/by-name/gt/gtypist/package.nix
+++ b/pkgs/by-name/gt/gtypist/package.nix
@@ -18,7 +18,7 @@ stdenv.mkDerivation (finalAttrs: {
     hash = "sha256-ymGAVOkfHtXvBD/MQ1ALutcByVnDGETUaI/yKEmsJS0=";
   };
 
-  CFLAGS = "-std=gnu99";
+  env.CFLAGS = "-std=gnu99";
 
   nativeBuildInputs = [ makeWrapper ];