error_wrapping in active_model_helper and nil errors

I'm running into an issue with MongoMapper and Rails 3. The problem occurs in actionpack/lib/action_view/helpers/active_model_helper.rb in the error_wrapping method. The problem is that when it gets to "object.errors[@method_name].any?", there are no errors present, and it throws the following error:

You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.any?

I was able to get around this problem by changing that line to this:

if object.respond_to?(:errors) && object.errors.respond_to?(:full_messages) && object.errors.present? && object.errors[@method_name].any?