转换为值类型'System.Int32'失败，因为实现的值为null

Here is my Home Controller:

这是我的家庭控制器:

using System;
using System.Collections.Generic;
using System.Data.Entity;
using System.IO;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using Biblioteca.Models;
using CrystalDecisions.CrystalReports.Engine;

namespace Biblioteca.Controllers
{
    public class HomeController : Controller
    {
        private BibliotecaDatabase db = new BibliotecaDatabase();

        public ActionResult Index()
        {
            var totalesviewmodel = new TotalesViewModel();
            totalesviewmodel.MontoCopias = db.AlumnosList.Sum(o => o.Copias);
            totalesviewmodel.MontoImpresiones = db.AlumnosList.Sum(o => o.Impresiones);
            totalesviewmodel.DineroDeposito = db.AlumnosList.Sum(o => o.Deposito);
            totalesviewmodel.DineroSap = db.AlumnosList.Sum(o => o.Sap);
    // *** here is my issue ***
            totalesviewmodel.MontoCopiasMaestro = db.MaestrosList.Sum(o => o.Copias);
            totalesviewmodel.MontoImpresionesMaestro = db.MaestrosList.Sum(o => o.Impresiones);
            return View(totalesviewmodel);
        }

        public ActionResult Reports()
        {
            List<Alumno> allDatos = new List<Alumno>();
            using (BibliotecaEntities dc = new BibliotecaEntities())
            {
                allDatos = dc.Alumnos.ToList();
            }
            return View(allDatos);
        }

        public ActionResult ExportReport()
        {
            List<Alumno> allDatos = new List<Alumno>();
            using (BibliotecaEntities dc = new BibliotecaEntities())
            {
                allDatos = dc.Alumnos.ToList();

                ReportDocument rd = new ReportDocument();
                rd.Load(Path.Combine(Server.MapPath("~/Reports"), "rpt_datos.rpt"));
                rd.SetDataSource(allDatos);

                Response.Buffer = false;
                Response.ClearContent();
                Response.ClearHeaders();

                try
                {
                    Stream stream = rd.ExportToStream(CrystalDecisions.Shared.ExportFormatType.PortableDocFormat);
                    stream.Seek(0, SeekOrigin.Begin);
                    return File(stream, "application/pdf", "Reporte.pdf");
                }
                catch (Exception ex)
                {
                    throw;
                }
            }
        }

        public ActionResult Contact ()
        {
            ViewBag.Message = "Your contact page.";
            return View();
        }
    }
}
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