Providing optional: true and shallow: true together produces an error

[contentfree]

I have

class ConversationsController < InheritedResources::Base
  belongs_to :campaign, :subscriber, optional: true, shallow: true
end

but this leads to an error

NameError:
       `@' is not allowed as an instance variable name

     # /usr/local/bundle/gems/inherited_resources-1.13.1/lib/inherited_resources/belongs_to_helpers.rb:79:in `instance_variable_defined?'
     # /usr/local/bundle/gems/inherited_resources-1.13.1/lib/inherited_resources/belongs_to_helpers.rb:79:in `get_parent_ivar'
     # /usr/local/bundle/gems/inherited_resources-1.13.1/lib/inherited_resources/belongs_to_helpers.rb:74:in `evaluate_parent'
     # /usr/local/bundle/gems/inherited_resources-1.13.1/lib/inherited_resources/base_helpers.rb:131:in `block in association_chain'
     # /usr/local/bundle/gems/inherited_resources-1.13.1/lib/inherited_resources/base_helpers.rb:130:in `each'
     # /usr/local/bundle/gems/inherited_resources-1.13.1/lib/inherited_resources/base_helpers.rb:130:in `inject'
     # /usr/local/bundle/gems/inherited_resources-1.13.1/lib/inherited_resources/base_helpers.rb:130:in `association_chain'
     # /usr/local/bundle/gems/inherited_resources-1.13.1/lib/inherited_resources/base_helpers.rb:173:in `end_of_association_chain'

This a polymorphic and optional relationship. I'm hoping to be able to do this:

• Visiting /conversations loads all conversations
• Visiting /campaigns/1/conversations loads all of this campaign's conversations
• Visiting /subscribers/1/conversations loads all of this subscriber's conversations
• edit_resource_path with Conversation:1 at any of the above pages should always and only produce /conversations/1/edit
• resource_path with Conversation:1 at any of the above pages should always and only produce /conversations/1