RSpec controller test: How do I check @organization.destroy is being called?

I thought it might be like:

let(:organization) { mock_model(Organization).as_null_object }

before(:each) do
  Organization.stub(:find).and_return(organization)
end

it "calls the destroy action on @organization" do
  assigns[:organization].should_receive("destroy")
  post :destroy, :id => organization.id
end

..but I get a "can't modify frozen object" error.

Here's how I would write that spec:

describe 'Oragnization#destroy' do
  let(:organization) { mock_model(Organization, :id => 1, :destroy => true) }

  subject { post :destroy, :id => organization.id }

  it { should be_successful }
end