trying to preview a crystal report in pdf format in web application by passing parameter but showing error message :

crReportDocument.Export()
missing parameter value

<CODE>

Dim crExportOptions As ExportOptions
Dim crDiskFileDestinationOptions As DiskFileDestinationOptions
Dim Fname As String

crReportDocument.Load(Session("rptFileName"))
CrystalReportViewer1.ReportSource = crReportDocument
CrystalReportViewer1.RefreshReport()
CrystalReportViewer1.ParameterFieldInfo.Clear()


Fname = "C:\Temp\Temp.pdf"
crDiskFileDestinationOptions = New DiskFileDestinationOptions()
crDiskFileDestinationOptions.DiskFileName = Fname
crExportOptions = crReportDocument.ExportOptions
With crExportOptions
.DestinationOptions = crDiskFileDestinationOptions
.ExportDestinationType = ExportDestinationType.DiskFile
.ExportFormatType = ExportFormatType.PortableDocFormat
End With

''''''''''''''''''''
If Session("rptType") = "PrintOption0" Then ' Without Parameter
' no need to do anything
ElseIf Session("rptType") = "PrintOption1" Then ' One Parameter
paramField.Name = "@UserID"
paramDiscreteValue1.Value = Session("rptParameter1")
paramField.CurrentValues.Add(paramDiscreteValue1)
paramFields.Add(paramField)
CrystalReportViewer1.ParameterFieldInfo = paramFields

End If
''''''''''''''''''''''

crReportDocument.Export()

Response.ClearContent()
Response.ClearHeaders()
Response.ContentType = "application/pdf"
Response.WriteFile(Fname)
Response.Flush()
Response.Close()

' delete the exported file from disk
System.IO.File.Delete(Fname)

</CODE>