Problem of respond_with working on nested resources

Rails
1

hi all:
when using namespace or nested resources in routes.rb,such as
"namespace :admin { resources :books }, we can use [:admin,@book] way to
generate path. It works fine in form_for , but when I use it with
respond_with in controller, I get problems.
The problem is when I put following code in controller:

    def create
     @book=Book.find(params[:id])
     @book.save
    respond_with[:admin,@book]
   end

if there are some problems and the @book is not saved, the function

should render new action, hence we can get error messages, but now it
render index action. and the log file said it got a error 302. if I
replace
‘respond_with [ :admin,@book]’ with 'respond_with
@book,:location=>admin_book_path(@book)", it works fine.
should we not use “[:admin,@book]” way with respond_with?

thanks