controller/ URL not working without controller/index

I'm following a book. It tells me to use: map.resource :controllername
in routes

But, once I do that, for some reason my main controller/index doesn't
work if I don't have "/index" behind it.

Anyone know why that would be? It just gives me:

Unknown action

No action responded to show. Actions: create, edit, index, and new

I think I figured it out.

I needed to have: map.resources :portfolios

NOT: map.resource :portfolios

(plural, not singular resources)