Error While Using Reports As A Web Service

I am using the Reports as a web service from Reporting Service

I am getting the following error on the Render menthod of ReportingService class

System.Web.Services.Protocols.SoapException was unhandled by user code
Message="System.Web.Services.Protocols.SoapException: An internal error occurred on the report server. See the error log for more details. ---> Microsoft.ReportingServices.Diagnostics.Utilities.InternalCatalogException: An internal error occurred on the report server. See the error log for more details. ---> System.NullReferenceException: Object reference not set to an instance of an object. at Microsoft.ReportingServices.ReportProcessing.ParameterInfoCollection.ToUrl(NameValueCollection coll) at Microsoft.ReportingServices.Library.RSService.RenderFirst(CatalogItemContext reportContext, ClientRequest session, Warning[]& warnings, ParameterInfoCollection& effectiveParameters, String[]& secondaryStreamNames) at Microsoft.ReportingServices.Library.RenderFirstCancelableStep.Execute() at Microsoft.ReportingServices.Diagnostics.CancelablePhaseBase.ExecuteWrapper() --- End of inner exception stack trace --- at Microsoft.ReportingServices.Diagnostics.CancelablePhaseBase.ExecuteWrapper() at Microsoft.ReportingServices.Library.RenderFirstCancelableStep.RenderFirst(RSService rs, CatalogItemContext reportContext, ClientRequest session, JobType type, Warning[]& warnings, ParameterInfoCollection& effectiveParameters, String[]& secondaryStreamNames) at Microsoft.ReportingServices.WebServer.ReportingService.Render(String Report, String Format, String HistoryID, String DeviceInfo, ParameterValue[] Parameters, DataSourceCredentials[] Credentials, String ShowHideToggle, Byte[]& Result, String& Encoding, String& MimeType, ParameterValue[]& ParametersUsed, Warning[]& Warnings, String[]& StreamIds) --- End of inner exception stack trace --- at Microsoft.ReportingServices.WebServer.ReportingService.Render(String Report, String Format, String HistoryID, String DeviceInfo, ParameterValue[] Parameters, DataSourceCredentials[] Credentials, String ShowHideToggle, Byte[]& Result, String& Encoding, String& MimeType, ParameterValue[]& ParametersUsed, Warning[]& Warnings, String[]& StreamIds)"
Source="ReportSampleForWebService"
Actor="http://gshah755/ReportServer/ReportService.asmx"
Lang=""
Node="http://gshah755/ReportServer/ReportService.asmx"
Role=""
StackTrace:
at ReportSampleForWebService._Default.Page_Load(Object sender, EventArgs e) in E:GunjanPracticeProjectsReportSampleForWebServiceReportSampleForWebServiceDefault.aspx.cs:line 110
at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e)
at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e)
at System.Web.UI.Control.OnLoad(EventArgs e)
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)


This is my code


protected void Page_Load(object sender, EventArgs e)

{

try

{





ReportingService rs = new ReportingService();

rs.Credentials = System.Net.CredentialCache.DefaultCredentials;

// Render arguments

byte[] result = null;

string reportPath = "/Report Project1/OrderDetails";

string format = "MHTML";

string historyID = null;





ParameterValue[] optionalParams = new ParameterValue[5];

ParameterValue p1 = new ParameterValue();

p1.Name = "ProductGroupID";

p1.Value = "Lunesta";

optionalParams[0] = new ParameterValue();

optionalParams[0].Name = "ProdGroupID";

optionalParams[0].Name = "Lunesta";



optionalParams[1] = new ParameterValue();

optionalParams[1].Name = "BeginDate";

optionalParams[1].Name = "02/01/2008";



optionalParams[2] = new ParameterValue();

optionalParams[2].Name = "EndDate";

optionalParams[2].Name = "02/29/2008";



optionalParams[3] = new ParameterValue();

optionalParams[3].Name = "HierarchyString";

optionalParams[3].Name = " ";



optionalParams[4] = new ParameterValue();

optionalParams[4].Name = "Level";

optionalParams[4].Name = "5";

DataSourceCredentials[] credentials = new DataSourceCredentials[1];

string showHideToggle = null;

string encoding;

string mimeType;

Warning[] warnings = null;

ParameterValue[] reportHistoryParameters = null;

string[] streamIDs = null;

SessionHeader sh = new SessionHeader();

rs.SessionHeaderValue = sh;









result = rs.Render(reportPath, format, historyID, "<DeviceInfo><StreamRoot>/RSWebServiceXS/</StreamRoot></DeviceInfo>", optionalParams, credentials,

showHideToggle, out encoding, out mimeType, out reportHistoryParameters, out warnings,

out streamIDs);







Response.BinaryWrite(result);

}

catch (Exception ex)

{

throw ex;

}

}

where I am doing wrong??