ruby-on-rails - RABL template with nil collection

Question

I have the following RABL template file

node(:status) { response.status }
node(:errors)  { @errors.inspect }

collection :@substances
attributes :name

Now my controller action isn't actually invoked, because an authentication filter is running before hand and setting the return status to 401. So @substances is nil. RABL then fails with a

undefined method `name' for nil:NilClass

error. How can I get RABL to not start iterating over the collection when it's nil?