Rails - Aug 2006 - Couldn''t find Producttype without an ID

I am at a lost for what is going on here-------I have a producttypes and
categories table.  prodtype_id is a foreign key in categories table that
references producttypes table.  Below is how I am tyying to display it
and get the above error.  Any suggestion

</p>
    <b>Product type:</b><br/>
    <select name="Category[prodtype_id]">
      <% @producttype.each do |producttype| %>
      <option value="<%= producttype.id %>">
        <%= producttype.producttype_name %>
      </option>
      <% end %>
    </select>
  </p>


Error is:  Couldn''t find Producttype without an ID