Error with default sorting on relationship field

Hello!
I'm using version 0.8.1 (also Rails 4.2.5.1 and Ruby 2.1.10) and following code

class A < Resource
    attributes :name
    has_one :b

    def self.default_sort
      [{field: 'b.name', direction: :asc}, {field: 'name', direction: :asc}]
    end 
end

class B < Resource
    attributes :name
end

results with following error, when I'm trying to make any request to A's resource.

ERROR -- : Internal Server Error: undefined method `model' for #<Class:0x000000042533a8> /vendor/bundle/ruby/2.1.0/gems/activerecord-4.2.5.1/lib/active_record/dynamic_matchers.rb:26

It is happening in apply_sort method, when try to get records.model.to_s:

associations = _lookup_association_chain([records.model.to_s, *model_names])

Is it a bug or something is wrong with my code?

Reproduce the request with the A and B resources, Rails 4.2.5.1, Ruby 2.1.10, and jsonapi-resources 0.8.1. Inspect apply_sort and the _lookup_association_chain call around records.model to determine whether relationship-field sorting is supported; done means the request no longer raises the reported undefined-method error or the limitation is clearly established.