diff --git a/pkgs/development/libraries/libmpeg2/default.nix b/pkgs/development/libraries/libmpeg2/default.nix
index 321204a0c4fd..2351ce2b5549 100644
--- a/pkgs/development/libraries/libmpeg2/default.nix
+++ b/pkgs/development/libraries/libmpeg2/default.nix
@@ -10,7 +10,7 @@ stdenv.mkDerivation rec {
   };
 
   # Otherwise clang fails with 'duplicate symbol ___sputc'
-  buildFlags = stdenv.lib.optionalString stdenv.isDarwin "CFLAGS=-std=gnu89";
+  buildFlags = stdenv.lib.optional stdenv.isDarwin "CFLAGS=-std=gnu89";
 
   meta = {
     homepage = http://libmpeg2.sourceforge.net/;