diff --git a/pkgs/by-name/li/libspf2/package.nix b/pkgs/by-name/li/libspf2/package.nix
index cd77d8072305..3873102de1b6 100644
--- a/pkgs/by-name/li/libspf2/package.nix
+++ b/pkgs/by-name/li/libspf2/package.nix
@@ -28,7 +28,7 @@ stdenv.mkDerivation (finalAttrs: {
       -e '/bin_PROGRAMS/s/spf_example_static//' src/spf_example/Makefile.am
   '';
 
-  CFLAGS = "-Wno-error=implicit-function-declaration";
+  env.CFLAGS = "-Wno-error=implicit-function-declaration";
 
   doCheck = true;