SSRS 2008 R2 call reporting web service "ListSubscription", but can not get Last Executed Date (it is blank)

Hi,

I am creating a report to monitor the SSRS subscription. I am using XML Web Services as a data source in my report.

My datasource is  http://yourSSRSserver/ReportServer/ReportService2005.asmx

My dataset query is 

<Query xmlns="http://schemas.microsoft.com/sqlserver/2005/06/30/reporting/reportingservices">
<Method Namespace="http://schemas.microsoft.com/sqlserver/2005/06/30/reporting/reportingservices" Name="ListSubscriptions"/>
<ElementPath>ListSubscriptionsResponse/SubscriptionItems/Subscription</ElementPath>
</Query>

The dataset return me below information 

SubscriptionID
  Owner
  Path
  VirtualPath
  Report
  DeliverySettings 
  Description
  Status
  Active 
  LastExecuted 
  ModifiedBy
  ModifiedDate
  EventType
  IsDataDriven

I can see all fields return with information except the "LastExecuted".

Did anyone know why and how to fix it?

Thanks,

Simon