diff --git a/pkgs/development/ruby-modules/gem-config/default.nix b/pkgs/development/ruby-modules/gem-config/default.nix
index b5aa0933c2e8..6f22b30f35fc 100644
--- a/pkgs/development/ruby-modules/gem-config/default.nix
+++ b/pkgs/development/ruby-modules/gem-config/default.nix
@@ -191,6 +191,10 @@ in
         [ darwin.apple_sdk.frameworks.CoreServices ];
   };
 
+  iconv = attrs: {
+    buildFlags = lib.optional stdenv.isDarwin "--with-iconv-dir=${libiconv}";
+  };
+
   # disable bundle install as it can't install anything in addition to what is
   # specified in pkgs/applications/misc/jekyll/Gemfile anyway. Also do chmod_R
   # to compensate for read-only files in site_template in nix store.