diff --git a/pkgs/by-name/kn/knot-dns/package.nix b/pkgs/by-name/kn/knot-dns/package.nix
index 6919ba9ec52d..f76750056487 100644
--- a/pkgs/by-name/kn/knot-dns/package.nix
+++ b/pkgs/by-name/kn/knot-dns/package.nix
@@ -98,7 +98,7 @@ stdenv.mkDerivation (finalAttrs: {
   enableParallelBuilding = true;
   strictDeps = true;
 
-  CFLAGS = [
+  env.CFLAGS = toString [
     "-O2"
     "-DNDEBUG"
   ];