Topic: Rspec not receiving unexpected message

I am trying to do this in my controller spec:

ProductType.should_receive(:find).and_return(@product_type)
get 'index', :product_type_id => @ferry.id

with this code in my controller

    @product_type = ProductType.find(params[:product_type_id]) unless params[:product_type_id].nil?
    puts "I am here"

And it works. But when I take out the should_receive method call it STILL works. I think it should be giving me a "received unexpected message" error, but it's not. It's very hard to test blindly.