[Rails] nil can't be coerced into BigDecimal

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!

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