We got the following error when we didn't specify the action:

Failure/Error: click_button "Save"
     AbstractController::ActionNotFound:
       The action 'create' could not be found for PostsController
     # ./spec/requests/admin_spec.rb:39:in `block (4 levels) in <top (required)>'
     # ./spec/requests/admin_spec.rb:35:in `block (3 levels) in <top (required)>'