`permitted_to?` in helper causing exception with rspec using `declarative_authorization`

I am writing rspec for helper. But getting error while using `permitted_to?' in helper.

It works fine in application but in test case it causing error.

Code

module PostModule
  module ApplicationHelper
    def link_for_post(post)
      if can_edit_post?(post)
        edit_post_path(post)
      else
        posts_path
      end
    end

    def can_edit_post?(post)
      permitted_to?(:edit, post, :context => :post_module_posts)
    end
  end
end

Rspec

describe PostModule::ApplicationHelper do
  describe '#link_for_post' do
    let(:user) { FactoryGirl.create(:user) }
    let(:first_post) {stub_model(PostModule::Post, {id: 1 })}
    it "should return edit link if has permission" do
      can_edit_post?(post).and_return(true)
      link_for_post(post).should eq('/posts/1/edit')
    end
    it "should return index path" do
      can_edit_post?(post).and_return(false)
      link_for_post(post).should eq('/posts')
    end
  end
end

Error

Failure/Error: can_edit_post?(post).and_return(true)
 NoMethodError:
   undefined method `permitted_to?' for #<RSpec::Core::ExampleGroup::Nested_1::Nested_1:0x0000001209b8e0>