Rails 3 dynamic form errors in boostrap 3

I'm using #403 dynamic forms Railcast combined with twitter-boostrap-rails 3 and simpleform gems. I have some custom validations for the dynamic form. For example when one field its empty I use:

self.errors.add field, "can't be blank"

This is error can be showed iterating over object.errors.full_messages. The problem its than boostrap 3 doesn't add the proper css and doen't add the text field can't be blank under the input. What can be causing this issue?