Silverlight -CRM2011:货币不能为空

我使用Silverlight o-data服务从我的应用程序与crm 2011进行交互当我尝试在实体SalesOrder中保存数据时，如下所示:

Private void beginSave()
{
SalesOrder orderHeader = new SalesOrder();
orderHeader.TransactionCurrencyId = new EntityReference(){ Id = new Guid("77D695B5-ACB4-E111-97BC-00155D55B216"), LogicalName="transactioncurrency" };
orderHeader.AccountId = new EntityReference() { Id = new Guid(MyClassGeneralOrder.customerId), LogicalName = "account" };
orderHeader.Name = "My Name";
Money totalAmount = new Money(); Money totalAmountBase = new Money(); 
Money totalTaxe = new Money(); Money totalAmountLessFreight = new Money();
totalAmount.Value = (decimal)MyClassGeneralOrder.InvoiceTotal;
totalAmountBase.Value = (decimal)MyClassGeneralOrder.totalRetail;
totalTaxe.Value = (decimal)MyClassGeneralOrder.totalCharges;
totalAmountLessFreight.Value = (decimal)MyClassGeneralOrder.totalNet;
orderHeader.TotalAmount = totalAmount;
orderHeader.TotalAmount_Base = totalAmountBase;
orderHeader.TotalTax = totalTaxe;
orderHeader.TotalAmountLessFreight = totalAmountLessFreight;
orderHeader.Description = element.Name;
orderHeader.PriceLevelId = new EntityReference() { Id = new Guid("03C5C4CB-EBD0-E111-8140-00155D55B216"), LogicalName="pricelevel" };
_context.AddToSalesOrderSet(orderHeader);
_context.BeginSaveChanges(SaveCallback, orderHeader);
}

private void SaveCallback(IAsyncResult result)
{
_context.EndSaveChanges(result);
}

在我的函数EndSaveChanges (result)中，我收到这个错误消息::«The Currency Cannot Be null»。我不明白为什么，因为我的"orderHeader。TransactionCurrencyId"字段不为空