The cast to value type 'System.Int32' failed because the materialized value is null

I have a Issue that I don't know what it is, there is my Home Controller:

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);
// there 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();
        }
    }
}

So, I don't know why I have this issue on:

totalesviewmodel.MontoCopiasMaestro = db.MaestrosList.Sum(o  => o.Copias );

before install crystal reports I don't have this issue, can any one help me?

What am I doing wrong?