diff --git a/pkgs/development/python-modules/kiwisolver/default.nix b/pkgs/development/python-modules/kiwisolver/default.nix
index 0a985bc3dee0..1481bc8699c0 100644
--- a/pkgs/development/python-modules/kiwisolver/default.nix
+++ b/pkgs/development/python-modules/kiwisolver/default.nix
@@ -20,7 +20,9 @@ buildPythonPackage rec {
     hash = "sha256-1BmXUZ/LpKHkbrSi/jG8EvD/lXsrgbrCjbJHRPMz6VU=";
   };
 
-  env.NIX_CFLAGS_COMPILE = lib.optionalString stdenv.isDarwin "-I${lib.getDev libcxx}/include/c++/v1";
+  NIX_CFLAGS_COMPILE = lib.optionals stdenv.isDarwin [
+    "-I${lib.getDev libcxx}/include/c++/v1"
+  ];
 
   nativeBuildInputs = [
     setuptools-scm