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

Product type:
<% @producttype.each do |producttype| %> <%= producttype.producttype_name %> <% end %>

Error is: Couldn’t find Producttype without an ID