显示错误值

namespace foo
{
    public class Personnes
    {
        string[] m_Noms;
        int m_NbElt;
        int m_Max;

        public Personnes(int Max)
        {
            m_Max = Max;
            m_NbElt = 0;
            m_Noms = new string[Max];
        }

        public int this[string Nom]
        {
            get { return Array.IndexOf(m_Noms, Nom); }
        }

        public string this[int i]
        {
            get { return m_Noms[i]; }
            set { m_Noms[i] = value;m_NbElt++; }
        }
    }
    class Prog
    {
        static void Main(string[] args)
        {
            Personnes Tableau = new Personnes(4);
            Tableau[0] = "Anna";
            Tableau[1] = "Ingrid";
            Tableau[2] = "Maria";
            Tableau[3] = "Ulrika";
            Console.WriteLine(Tableau[1]); 
            Console.WriteLine(Tableau["Maria"]);
            Console.WriteLine(Tableau[10]); 
            Console.WriteLine(Tableau["Toto"]); 


        }
    }
}

我听说了 Console.WriteLine(Tableau[10]);