diff --git a/pkgs/by-name/le/lepton-eda/package.nix b/pkgs/by-name/le/lepton-eda/package.nix
index e685ec889d41..6e2895785fca 100644
--- a/pkgs/by-name/le/lepton-eda/package.nix
+++ b/pkgs/by-name/le/lepton-eda/package.nix
@@ -52,7 +52,7 @@ stdenv.mkDerivation rec {
     "--with-gtk3"
   ];
 
-  CFLAGS = [
+  env.CFLAGS = toString [
     "-DSCM_DEBUG_TYPING_STRICTNESS=2"
   ];