`acts_as_nested_set': undefined local variable or method `accessible_attributes'

[arlyxiao]

I'm developing another gem based on this gem.
when added the code 'acts_as_nested_set' to model, and run rspec spec,
then the error appears

/.rvm/gems/ruby-2.1.3/gems/mongoid_nested_set-0.2.1/lib/mongoid_nested_set/base.rb:56:in acts_as_nested_set': undefined local variable or methodaccessible_attributes' for MPNestedSet::Category:Class (NameError)