fields_for :index problem

[laurynas]

I have a problem passing custom :index to fields_for, like
<%= f.fields_for :item, item, :index => 'someindex' do |fields| %>
for all fields it works fine except calendar_date_select

If I pass :index to calendar_date_select it works fine:
<%= fields.calendar_date_select :date, :index => 'someindex' %>