I also happened to bump into this problem, see https://sourceforge.net/mailarchive/forum.php?thread_name=aac3cf150908311026h3cfa7078p67f3d52bb7b7293%40mail.gmail.com&forum_name=rubycocoa-talk

Though I haven't gotten any feedback on the issue I came to the conclusion that maybe subclasses of NSObject aren't allowed to have optional arguments, because that's the way Obj-C works. Maybe should try talking too #ruby-osx on IRC?