SSRS 2008 Render to XML - Where are my Chart Streams?

Question

I'm using code like this:

private byte[] RenderReport(ReportOptions reportOptions, string format, string deviceInfo, out string[] streamIds)
 {
  byte[] result;
  string empty;

  ReportExecutionService.ParameterValue[] parameters = reportOptions.Parameters;
  ReportExecutionService.Warning[] optionalWarnings;

  _reportExecutionService.LoadReport(reportOptions.ReportPath, null);
  _reportExecutionService.SetExecutionParameters(parameters, null);

  result = _reportExecutionService.Render(
   format,
   deviceInfo,
   out empty,
   out empty,
   out empty,
   out optionalWarnings,
   out streamIds
   );

  return result;
 }

The issue is that streamIds never gets populated with anything, even though I have a chart in my report. I'd expect it to get populated with the ID to pass to RenderStream and generate my pie chart image? I'm pretty sure I've done this successfully on SSRS 2005 so I'm not sure why it wouldn't work here.