NullReferenceException in ViewBag

我的ViewBag线抛出一个NullReferenceException:

public ActionResult KnowledgebaseSuggestions(String IncidentTags)
    {
        KnowledgeService KS = new KnowledgeService(db);
        ViewBag.KSResults = KS.GetSuggestionsByTags(IncidentTags.Split(',').Select(t => t.Trim()).ToList());
        return View();
    }

getadvistionsbytags方法是:

   private readonly db_SLee_FYPContext db;
    public KnowledgeService(db_SLee_FYPContext db)
    {
        this.db = db;
    }

    public IEnumerable<KnowledgebaseViewModel> GetSuggestionsByTags(IEnumerable<string> tags)
    {
        var validSuggestions = db.Knowledgebases.Where(k => tags.Any(t => k.KnowledgebaseTitle.Contains(t)))
                                    .Select(k => new KnowledgebaseViewModel()
                                    {
                                        ID = k.KnowledgebaseID,
                                        Title = k.KnowledgebaseTitle,
                                        Link = k.KnowledgebaseLink
                                    })
                                    .ToList();
        return validSuggestions;
    }

我不确定NullReference是指什么，或者为什么它首先是一个NullReference ?