程序里面有个数据列表页，动态生成相关数据的二维码，现在程序抛出异常如下：

A generic error occurred in GDI+.

                        Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.            

            Exception Details: System.Runtime.InteropServices.ExternalException: A generic error occurred in GDI+.

生成二维码的程序：

  public string ServiceQRCode(int tId)
        {

            string qrcode = Server.MapPath("/download/" + agentId + ".jpg");
            string url = Request.Url.Scheme + "://" + Request.Url.Host + "/" + tId;
            QRCodeHelper.GetQRCode(url, 140, qrcode);
            return "/download/" + agentId + ".jpg";
        }