nil can't be coerced into BigDecimal Options

hi guys im playing with RoR with the environment of rails 3 ruby 1.9 i got stuck in nil can't be coerced into BigDecimal error i need to get the total cost of the products inside the cart i know where the problem is(i think) but i almost did every thing cart/show.html.rb <div class="cart_title" >Your Cart</div>     <table>         <% for item in @cart.line_items %>     <tr>         <td><%= item.quantity %>&times;</td>         <td><%= item.product.title %></td>     <td class="item_price" ><%= number_to_currency(item.total_price) %></td>     </tr>     <% end %>         <tr class="total_line" >         <td colspan="2" >Total</td>         <td class="total_cell" ><%= number_to_currency(@cart.total_price) %></td>     </tr>     </table>         <%= button_to 'Empty cart', @cart, :method => :delete,         :confirm => 'Are you sure?' %> model/line_item.rb   def total_price     line_items.to_a.sum { |item| item.total_price }   end model/cart.rb def total_price     product.price * quantity   end my second option is def total_price      if product.price        product.price * quantity      else        product.price = "0.0".to_d      end    end but still this wont work thanks more power to us!