By default, the EmailService passes an order guid to the email template. But when I try to query the purchase orders by guid I get a 'GenericADOException': "could not execute query". So how am I supposed to retrieve the PurchaseOrder with just a guid?

Here is my code:

string orderNumber = Request.QueryString["orderNumber"];
PurchaseOrder purchaseOrder = PurchaseOrder.FirstOrDefault( x => x.OrderGuid.ToString() == orderNumber ); // Blows up here

But it works if I send the email manually and pass an order number:

// Send order confirmation email manually
EmailProfile emailProfile = SiteContext.Current.CatalogContext.CurrentCatalogSet.EmailProfile;
var emailService = new EmailService();
var mailAddress = new MailAddress( order.GetBillingAddress().EmailAddress );
string orderId = order.OrderId.ToString();
string s = emailService.Send( emailProfile, "OrderConfirmation", mailAddress, orderId );

// Get order details
string orderNumber = Request.QueryString["orderNumber"];
PurchaseOrder purchaseOrder = PurchaseOrder.FirstOrDefault( x => x.OrderId.ToString() == orderNumber );