当我有这样的控制器时：

class PostsController < InheritedResources::Base
end

我用rspec测试以下内容

describe "PUT update" do
    describe "with invalid params" do
      it "re-renders the 'edit' template" do
        post = Post.create! valid_attributes
        # Trigger the behavior that occurs when invalid params are submitted
        Post.any_instance.stub(:save).and_return(false)
        put :update,{:id => post.to_param,:post => {}},valid_session
        response.should render_template("edit")
      end
    end
  end

我收到以下错误：

3) PostsController PUT update with invalid params re-renders the 'edit' template
     Failure/Error: response.should render_template("edit")
       expecting <"edit"> but rendering with <"">
     # ./spec/controllers/posts_controller_spec.rb:115:in `block (4 levels) in <top (required)>'

为什么是这样？我是否必须将其他东西存在？