Problem with accessing a variable in a nested form

I have a nested form in rails view called like this

<% f.fields_for :invoice_item_numbers do |item_no_form| %>
    <%= render 'invoice_item_number', :f => item_no_form %>
<% end %>

      

        
        
        
      

    

and the partial (_invoice_item_number.html.erb) looks like this:

<div class='invoice_item_numbers'>
<% if f.object.new_record? %>
        <li><%= f.label :item_number %><%= f.text_field :item_number %>
    <%= link_to_function "remove", "$(this).parent().remove()", :class => 'remove_link' %></li>
<% else %>
    <li class="inline"><%= f.label :item_number %><%= f.text_field :item_number %>
    </li><li class="inline"><%= f.label :description %><%= invoice_item_number.description %></li><li><%= f.label :amount %><%= f.text_field :amount %>
    <%= f.check_box '_destroy', :class => 'remove_checkbox' %>
    <%= f.label '_destroy', 'remove', :class => 'remove_label' %></li>
<% end %>
</div>

      

        
        
        
      

    

It fails with an error message

undefined method `description' for nil:NilClass

      

        
        
        
      

    

Why does invoice_item_number return null object in this partial? Obviously this is defined in some way, because if I change it to something else (for example item_number.description

, then the error message becomes undefined local variable or method

item_number 'for # instead. The invoice_item_number object that is being displayed by this partial is being used perfectly well by the form helpers as

<% = f.text_field: item_number%> and

<% f.text_field : number%> both work perfectly well. I have tried a number of solutions such as using

@ invoice_item_number` and explicitly defining the object in the render method, but they didn't work.

Presumably there is a very simple answer to this question.