Error with fields_from

Heya guys and gals.
I was following ryan bates complex forms #73 + #74 and have come
across a problem which i’m having trouble solving.

my _test.erb partial looks like

<% fields_for “story[acceptence_test][]” do |test_form| %>

Test: <%= test_form.text_field :test %>

<% end %>

When I try to load a page that calls this partial I get
`@story[acceptance_test]’ is not allowed as an instance variable name

If I remove the [] from the first line leaving me with only story
[acceptance_tests] the page loads fine, but I can’t save the form.

Any ideas?