Resourse route in namespace with prefix error

Ok so I have this route

map.namespace :trades do |trades|
    trades.resources :buys, :path_prefix => '/trades/:contract_id'
end

I want the contract_id variable to filter a list of buys

then I try to open this route http://localhost:3000/trades/1/buys

I get a strange error:

ActionController::RoutingError in Trades/buys#index

Showing trades/buys/index.html.erb where line #12 raised:

trades_buy_url failed to generate from {:contract_id=>#<Trades::Buy
id: 5951, asset_id: 309, type: "Buy", contract_id: 1, investor_id: 1,
date: "2001-09-30", trade_status_id: 1, is_future_operation: false,
quantity: #<BigDecimal:7fb8004,'0.2E2',4(8)>, price: #<BigDecimal:
7fb7fb4,'0.623E1',8(12)>, total: #<BigDecimal:7fb7f64,'0.1246E3',
8(12)>, created_at: "2008-06-10 22:49:31", updated_at: "2008-06-10
22:49:31">, :action=>"show", :controller=>"trades/buys"}, expected:
{:action=>"show", :controller=>"trades/buys"}, diff:
{:contract_id=>#<Trades::Buy id: 5951, asset_id: 309, type: "Buy",
contract_id: 1, investor_id: 1, date: "2001-09-30", trade_status_id:
1, is_future_operation: false, quantity: #<BigDecimal:7fb6a38,'0.2E2',
4(8)>, price: #<BigDecimal:7fb69e8,'0.623E1',8(12)>, total:
#<BigDecimal:7fb6998,'0.1246E3',8(12)>, created_at: "2008-06-10
22:49:31", updated_at: "2008-06-10 22:49:31">}

Extracted source (around line #12):

12: <td><%= link_to 'Show', buy %></td>

Is this a bug of some sort?

NOTE:
if I dont use named routes I dont get this error for exampleif I use

<td><%= link_to 'Show', {:controller => "buys", :action => "show", :id
=> buy.id} %></td>

There is no error