Trying to use a transform binding instead of a computed property

gistfile1.txt

App.ticketsController = Em.ArrayController.create({
  content: Em.A();
});


App.backlogController = Em.ArrayController.create({
  contentBinding: Em.Binding.transform(function(value, binding) {
    return value.filter(function(item, index, enumerable) {
      return (item.get('assigned') == undefined);
    });
  }).from("App.ticketsController.content")
});