c# passing date to crystalreportviewer not working

when I pass a string parameter &P1=text to Crystal Report viewer then the first prompt doesn't show, only the others and the report runs, as I want.. (see attached P1.jpg)

but when I send a date: &P2=01/01/12 the second parameter doesn't show, but when I click ok I am then prompted for the second parameter that I passed. (see attached P2a.jpg and P2b.jpg)

why are the passed dates being cleared and reprompted?

I use this URL:

http://127.0.0.1/MyReport/test.aspx?reportpath=test4.rpt&P2=01/01/2012

       private void ReportViewer()
        {
            String reportFile = Request.QueryString.Get("reportpath");

            CrystalReportViewer1.ReportSource = Server.MapPath(reportFile);
            string userID = "crystal";
            string password = "reports";

            foreach (TableLogOnInfo boTableLogonInfo in CrystalReportViewer1.LogOnInfo)
            {
                ConnectionInfo boConnectionInfo = boTableLogonInfo.ConnectionInfo;
                boConnectionInfo.UserID = userID;
                boConnectionInfo.Password = password;
            }

            // pass parameters, prompt for needed ones
            for (int i = 0; i < CrystalReportViewer1.ParameterFieldInfo.Count; i++)
                {
                    if (string.IsNullOrEmpty(Request.QueryString.Get("P" + (i + 1))))
                    {
                            //do nothing this parameter is not passed
                    }
                    else
                    {
                    ParameterField paramField = CrystalReportViewer1.ParameterFieldInfo[i];
                    ParameterDiscreteValue ParamDiscreteValue = new ParameterDiscreteValue();
                    ParamDiscreteValue.Value = Request.QueryString.Get("P" + (i + 1));
                    paramField.CurrentValues.Add(ParamDiscreteValue);
                    }
                }
        }