ArgumentNullException：值不能为空

如果数据库中有对象，为什么在这种情况下对象是 Audience == null？

    [Authorize(Roles = "Администратор")]
public class AudienceController : Controller {
    private readonly ApplicationContext db;

    public AudienceController(ApplicationContext context) {
        db = context;
    }
    public async Task<IActionResult> Occupy(MyViewModelSearch model) {

        if (model.SearchAuditMod != null) {

            MyViewModelSearch modelforsearch = new MyViewModelSearch {
                AudiencesMod = await db.Audiences.ToListAsync(),
            };

            foreach (Audience audience in modelforsearch.AudiencesMod) {
                if (audience.Building == model.SearchAuditMod.Building) {

                    model.AudiencesMod.Append(audience);
                }
            }
        }

        return View(model);
    }
}

视图模型：

public class MyViewModelSearch {
    public IEnumerable<Audience> AudiencesMod { get; set; }
    public SearchAuditViewModel SearchAuditMod { get; set; }
}

观众：

public class Audience {
    public int Id { get; set; }

    [Required]
    [Display(Name = "Корпус")]
    public string Building { get; set; } 

    [Required]
    [Display(Name = "Номер аудитории")]
    public int Number { get; set; } 

    public bool Projector { get; set; } 
    public bool Computer { get; set; } 
}

搜索审计视图模型：

public class SearchAuditViewModel {
    public DateTime Data { get; set; }
    public string Building { get; set; }
    public int min { get; set; }
    public int max { get; set; }
    public bool Projector { get; set; }
    public bool Computer { get; set; }
}