diff --git a/pkgs/development/python-modules/lxml/default.nix b/pkgs/development/python-modules/lxml/default.nix
index 093ec343a6db..fae582f17072 100644
--- a/pkgs/development/python-modules/lxml/default.nix
+++ b/pkgs/development/python-modules/lxml/default.nix
@@ -22,7 +22,9 @@ buildPythonPackage rec {
   nativeBuildInputs = [ libxml2.dev libxslt.dev cython ] ++ lib.optionals stdenv.isDarwin [ xcodebuild ];
   buildInputs = [ libxml2 libxslt zlib ];
 
-  env.NIX_CFLAGS_COMPILE = lib.optionalString stdenv.cc.isClang "-Wno-error=incompatible-function-pointer-types";
+  env = lib.optionalAttrs stdenv.cc.isClang {
+    NIX_CFLAGS_COMPILE = "-Wno-error=incompatible-function-pointer-types";
+  };
 
   # tests are meant to be ran "in-place" in the same directory as src
   doCheck = false;