Error Guest Anonymous checkout

In Anonymous checkout in page checkoutconfirm.aspx with any type of payment when I press the button confirm, i have this error: "NULLABLE OBJECT MUST HAVE A VALUE".

With Registration user is all ok!!.....

Where is the problem???

Thanks Marco

P.S I use NopCommerce 1.90 and visual studio 2010.

LOG:
System.Exception: System.InvalidOperationException: Nullable object must have a value. at System.Nullable`1.get_Value() at NopSolutions.NopCommerce.BusinessLogic.Messages.MessageService.ReplaceMessageTemplateTokens(Order order, String template, Int32 languageId) at NopSolutions.NopCommerce.BusinessLogic.Messages.MessageService.SendOrderPlacedStoreOwnerNotification(Order order, Int32 languageId) at NopSolutions.NopCommerce.BusinessLogic.Orders.OrderService.PlaceOrder(PaymentInfo paymentInfo, Customer customer, Guid orderGuid, Int32& orderId)


Help me.... is very important for me....


UPDATE: The problem is....,  I have put in MessageService.cs this two line.....
    
     DateTime data_nascita = order.Customer.DateOfBirth.Value;
     tokens.Add("Order.Nascita", HttpUtility.HtmlEncode(data_nascita.ToShortDateString()));

     In Registration user this two line print the date of birth of user in email store order....

    Datetime not accept nullable value.... how resolve???