Ember-CLI and Rails: No 'Access-Control-Allow-Origin' header error

I am trying to build an app with Ember-CLI (ember.js) and Rails 4.2.0. I am keeping the Ember and Rails codebases separate, so I have enabled CORS with the rack-cors gem. I keep getting the following errors when the Ember app makes a request to the Rails api:

406 Not Acceptable

And:

No 'Access-Control-Allow-Origin' header is present on the requested 
resource. Origin 'http://localhost:4200' is therefore not allowed access.

And this on the Rails side:

Processing by CustomersController#index as HTML
Completed 406 Not Acceptable in 2ms

ActionController::UnknownFormat (ActionController::UnknownFormat):
  app/controllers/customers_controller.rb:9:in `index'

The request Accept header is:

Accept:application/json, text/javascript, */*; q=0.01

And my controller action is:

  def index
    @customers = Customer.all

    respond_to do |format|
      format.json { render json: @customers }
      #format.js { render json: @customers } Tried this too
    end
  end

I also have a customer_serializer using the active_model_serializer gem.

Any help will be greatly appreciated!