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!