Mongoid STI scopes exception

mongoid_sti_scopes_exception.rb

require 'rubygems'
require 'mongoid'

connection = Mongo::Connection.new
Mongoid.config.database = Mongo::DB.new('example', connection)

class A
  include Mongoid::Document
  field :foo, :type => Integer

  scope :asc_by_foo, asc(:foo)
end

class B < A
  field :bar, :type => Integer

  scope :asc_by_bar, asc(:bar)
end

B.asc_by_foo # => NoMethodError: undefined method `conditions' for nil:NilClass