what does rendering with <[]> mean in rails 4?

What does <[]> exactly mean in rails 4 ?

I wrote a test for "redirect to the index page after user logout ". it did redirect to the index page when i operate on the browser. However i encountered the following error after i ran the rake test task in the terminal.

[/test/integration/user_stories_test.rb:18]:

expecting <"index"> but rendering with <[]>

integration test :

delete "/logout"
assert_response :redirect
assert_template "index"

session controller:

def destroy
    session[:user_id] = nil
    redirect_to store_url, notice: "Logged out"
  end
end