Programatically Passing null value to Crystal Report Viewer

Hi guys,

I have developed an application in VS 2010 that uses the Crystal Report Viewer. I programatically passing values to the reports. Many of the reports have optional prompts for their parameters. I would like to be able to run the report and send a blank or null value to the parameters. If I don't pass parameters the viewer prompts the user to enter them, if I pass null I get an exception that the types are not compatible. Can anyone help? Here is a sample code:

                                crParameterDiscreteValue.Value = null;

                                crParameterFieldDefinitions = cryRpt.DataDefinition.ParameterFields;

                                crParameterFieldDefinition = crParameterFieldDefinitions["@BPGroup"];

                                crParameterValues = crParameterFieldDefinition.CurrentValues;

                                crParameterValues.Add(crParameterDiscreteValue);

                                crParameterFieldDefinition.ApplyCurrentValues(crParameterValues);

thanks,

Costas