ReportViewer with null values

I generate a report with area, Type, and two dates within which the area and the type can contain or not values, with the four parameters works correctly, but how can I get the query executed correctly when the value is null? ? Thanks!

        //Array 
        ReportParameter[] parameters = new ReportParameter[4];
        //Establecemos el valor de los parámetros
        parameters[0] = new ReportParameter("Area", lbl1.Text);
        parameters[1] = new ReportParameter("Source", lbl2.Text);
        parameters[2] = new ReportParameter("fecha1",f11);
        parameters[3] = new ReportParameter("fecha2", f22);
        //array al ReportViewer
        ReportViewer1.LocalReport.SetParameters(parameters);
       ReportViewer1.LocalReport.Refresh();'

The parameters are extracted from three DropDownList