RDLC Report Error

Hi,
I encounter some problem in RDLC report.
When I create a clean project and add dataset and reports, it runs correctly and reports can be load.
However, when I add these report to our existing core project (.NET 4.0), it shows error in following statement:
byte[] renderedBytes = rv.LocalReport.Render("PDF"); And the error is:
{Microsoft.Reporting.WebForms.LocalProcessingException: An error occurred during local report processing. ---> Microsoft.ReportingServices.ReportProcessing.ProcessingAbortedException: An error has occurred during report processing. ---> Microsoft.ReportingServices.ReportProcessing.ReportProcessingException: DataSet1 at Microsoft.ReportingServices.OnDemandProcessing.RuntimeDataSet.RunDataSetQuery() at Microsoft.ReportingServices.OnDemandProcessing.TablixProcessing.RuntimeOnDemandDataSet.Process() at Microsoft.ReportingServices.OnDemandProcessing.RuntimeD ...