CR .NET API: Attempt to change property of group chart fails

I am using the CR .NET RAS API to work with chart objects in a report.

If the chart is in the Report Footer area I can successfully change its type and other properties with code like this:

   ChartObject newChartObject = (ChartObject)chartObject.Clone();

   newChartObject.ChartStyle.Type = (CrChartStyleTypeEnum)value;

   reportDefController.ReportObjectController.Modify(chartObject, newChartObject);

However, if the chart is in a Group Footer area I get an exception when I call Modify:

COMException: The summary data field of group chart does not exist.

In fact the summary field that the chart is based on is present as a field object in that same section.

Any insights?

- rick