fields_for does not accept an array as first argument

According to documentation I found (http://railsapi.com/doc/rails-v3.0.0RC/classes/ActionView/Helpers/F...) fields_for's first argument should accept an array. However I get an error pointing at line 1 in my HAML partial (fields_for is on line 1). I verified that @node is a valid model and not an array.

= fields_for [:admin, @node] do |form|
  ...

Gives me the following error:

undefined method `model_name' for Array:Class

Replacing fields_for with form_for works:

= form_for [:admin, @node] do |form|
  ...