collection_select doesn't honour :index option to fields_for

If I do

fields_for :post, @post, :index => 1 do |f|
  f.collection_select(:author_name, @posts, :author_name, :author_name)
end

I get

<select name="post[author_name]" ... >

Instead of the expected

<select name="post[1][author_name]" ... >

Without specifying index, it works correctly, i.e.

fields_for 'post[]', @post do |f|

does generate the correct <select name="post[1][author_name]"

The other helpers (text_field, checkbox etc) do work correctly with the :input option.