How to reference a shared datasource already on report server for data driven subscription

I see there's MS documentaion on creating a data driven subscription.
The example shows to create a data source for the data retrieval plan.
see
http://msdn2.microsoft.com/en-us/library/microsoft.wssux.reportingserviceswebservice.rsmanagementservice2005.reportingservice2005.createdatadrivensubscription.aspx
I would like to set the data source to a share data source on the
server. How should I implement that?
I have tried to use the following but failed:
...
delivery.Item = rs.GetDataSourceContents("/Data Sources/
testDataSource");
...I have worked this out: By creating a DataSourceReference object:
DataSourceReference reference = new
DataSourceReference();
reference.Reference = "/Data Sources/testDataSource";
delivery.Item = reference;
But then I got error like:
The current action cannot be completed because the user data source
credentials that are required to execute this report are not stored in
the report server database.
My credential information is stored with data source. If I manually
create the data-driven subscription from Report Manager, and use the
same shared data source, I wouldn't get this error. Anyone knows why?
Thanks