Parameter doesn't change its value after assigning an empty string

Hi,

Is there any way to make a parameter null when in the report designer as I have parameters that are optional in the stored procedure?
In crystal reports you can tick a null checkbox on the parameter when requesting data.

I am currently using the script below on a test report which allows me to retrieve the data with out errors from the datasource when I leave the optional parameter blank but if I try to put a value in to the text box and press submit it doesn't use it and when you click on the parameter panel again the value disappears.

I am using a custom parameter panel on a report viewer app I have created so I don't have these problems.

Thanks
Craig Smith

Visual Basic
Private Sub XtraReport_ParametersRequestSubmit(ByVal sender As Object, ByVal e As DevExpress.XtraReports.Parameters.ParametersRequestEventArgs)  

    Dim Info As Object = e  

        For Each Info In e.ParametersInformation  
        If Info.Parameter.Type.FullName = "System.String" Then  
            If Info.Parameter.Value = "" Then Info.Parameter.Value = DBNull.Value  
        End If  
    Next  

End Sub