how to create a FieldObject by RunningTotalField

I'm trying create FieldObject by RunningTotalField,but When an exception is thrown "no error"，please help me.thanks!

  // "{#RUN_CoCode}" is existing fields   RunningTotalField runTotalObj = (RunningTotalField)boReportClientDocument.DataDefController.FindFieldByFormulaForm("{#RUN_CoCode}");  to = new CrystalDecisions.ReportAppServer.ReportDefModel.FieldObject();  to.DataSourceName = runTotalObj.FormulaForm;  to.FieldValueType = runTotalObj.Type;   to.Left = 4 * 1440;   to.Width = 3 * 1440;     boReportClientDocument.ReportDefController.ReportObjectController.Add(to, section);

Message was edited by: Ludek Uher