Passing null param value to SSRS executionService 2005 in asp.net

Hi

is anyone knows how to pass SSRS report parameter as NULL to SSRSExecutionService2005 via code.

I am using SSRS web services to generate report and rendering its MHTML to aspx page.

PROBLEM: I am using below code

SSRSReportExecutionService2005.ParameterValue[] rptParameters = new SSRSReportExecutionService2005.ParameterValue[2];

                rptParameters[0] = new SSRSReportExecutionService2005.ParameterValue();
                rptParameters[0].Name = "Campaign";
                rptParameters[0].Value = campaignId.ToString();

                rptParameters[1] = new SSRSReportExecutionService2005.ParameterValue();
                rptParameters[1].Name = "Survey";
                rptParameters[1].Value = null;

                rsExec.SetExecutionParameters(rptParameters, "en-us"); 
                results = rsExec.Render(format, deviceInfo, out extension, out encoding, out mimeType, out warnings, out streamIDs);  // getting error Here                  Response.ContentType = "message/rfc822";
                Response.AddHeader("content-length", results.Length.ToString());
                Response.BinaryWrite(results);

                Response.Flush();
                HttpContext.Current.ApplicationInstance.CompleteRequest();

I am getting below exception:
This report requires a default or user-defined value for the report parameter 'Survey'

please help as soon as possible