将日期时间传递到报表查看器时，参数验证失败

所以我试图在asp中使用reportviewer控件查看远程ssrs报告。报告有三个参数——一个nvarchar和两个日期时间。传递单个nvarchar运行良好，直到我添加两个日期时间，此时我得到以下错误。所以肯定有什么问题。我总是得到可怕的错误:

    Parameter validation failed. It is not possible to provide valid values for all    parameters. (rsParameterError)

尽管这三个参数都传入了值。我猜这是日期字符串中/字符的问题，所以我尝试了url编码，但无济于事。我可以确认此报告确实从报告服务中运行，但我似乎无法从代码中正确地传递参数值。建议吗?

我的代码如下

    string firstdate = datepicker.Value;
    string enddate = datepicker2.Value;
    firstdate = HttpUtility.UrlEncode(firstdate);
     enddate = HttpUtility.UrlEncode(enddate);
    ReportParameter[] parm = new ReportParameter[3];
    parm[0] = new ReportParameter("store_name", DropDownList1.SelectedValue);
    parm[1] = new ReportParameter("mindate",firstdate);
    parm[2] = new ReportParameter("maxdate", enddate);

    ReportViewer1.ServerReport.ReportPath = "<REDACTED>";
    ReportViewer1.ServerReport.ReportServerUrl = new Uri("http://<REDACTED>/ReportServer");
    ReportViewer1.ServerReport.ReportServerCredentials = new ReportServerCredentials("<REDACTED>", "<REDACTED>", "");
    ReportViewer1.ProcessingMode = ProcessingMode.Remote;
    ReportViewer1.ServerReport.SetParameters(parm);
    ReportViewer1.ServerReport.Refresh();