When creating a rails ember app I get :frontend app is not defined, why is this?

I am trying to create a simple rails ember web app but I am having trouble.

When I run the following code:

rails generate ember:init

I get this error:

lib/ember_cli.rb:21:in `block in app': :frontend app is not defined (KeyError)

This is my routes.rb file:

Rails.application.routes.draw do mount_ember_app :frontend, to: "/" end

Can anyone see what I am doing wrong?

Thanks so much, Kato.