Crystal Report XI error. ( Unsupported Operation. A document processed by the JRC en)

Hi.
I have created a report using crystal report XI and populated the report using Database Expert (connecting to database).
Now, when I used that report in my c# program, i get the error

"Unsupported Operation. A document processed by the JRC engine cannot be opened in the C++ stack."

Here's the code I used to generate the report.

Code:

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string empclass = "Support"
string  dtFrom ="04/01/2014";
string  dtTo = "04/30/2014";
DataSet ds = new DataSet();
ds=tk.Summary_OnRowDataBound(dtFrom, dtTo, 1, empclass, 1); // this function returns a dataset.


ReportDocument rpt = new ReportDocument();
rpt.Load("~/Reports/TimeKeepingSummary.rpt");
rpt.Database.Tables[0].SetDataSource(ds.Tables[0]);
CrystalReportViewer1.ReportSource = rpt;

rpt.SetDataSource(ds);
System.Threading.Thread.Sleep(1000);
string exportedDocFileName = "TimeKeepingSummary_" + DateTime.Now.ToString();
rpt.SetDataSource(ds);
rpt.ExportToHttpResponse(ExportFormatType.Excel, Response, true, exportedDocFileName);

rpt.Close();
rpt.Dispose();

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Please help me with thi.s Thank you and have a nice day!