Error Durring XML Serialization

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml.Serialization;

namespace ClassLibrary1
{
    public class Program
    {
        static void Main(string[] args)
        {
            Library<string, string, string> Lib = new Library<string, string, string>();
            Lib.Subjects.Add(new LibrarySubject<string,string,string>("Test!"));
            Lib["Test!"].Entries.Add(new LibrarySubjectEntry<string,string>("Rawr!", "RARRR"));
            Console.WriteLine(Lib["Test!"]["Rawr!"]);
            System.IO.FileStream FS = System.IO.File.Create("Test.xml");
            XmlSerializer S = new XmlSerializer(Lib.GetType());
            S.Serialize(FS, Lib);
            FS.Flush();
            FS.Close();
        }
    }
    public class Library<TSubKey, TKey, TVal>
        where TSubKey : IComparable
        where TKey : IComparable
    {
        public Library()
        {
            Subjects = new List<LibrarySubject<TSubKey, TKey, TVal>>();
        }
        [XmlIgnore]
        public LibrarySubject<TSubKey, TKey, TVal> this[TSubKey SubjectKey]
        {
            get
            {
                return Subjects[Subjects.FindIndex((LibrarySubject<TSubKey, TKey, TVal> Subject) => Subject.SubjectKey.CompareTo(SubjectKey) == 0)];
            }
            set
            {
                Subjects[Subjects.FindIndex((LibrarySubject<TSubKey, TKey, TVal> Subject) => Subject.SubjectKey.CompareTo(SubjectKey) == 0)] = value;
            }
        }
        public List<LibrarySubject<TSubKey, TKey, TVal>> Subjects;
    }

    public class LibrarySubject<TSubKey, TKey, TVal>
        where TSubKey : IComparable
        where TKey : IComparable
    {
        public LibrarySubject(TSubKey SubjectKey)
        {
            this.SubjectKey = SubjectKey;
            this.Entries = new List<LibrarySubjectEntry<TKey, TVal>>();
        }
        [XmlIgnore]
        public TVal this[TKey Key]
        {
            get
            {
                return Entries[Entries.FindIndex((LibrarySubjectEntry<TKey, TVal> Entry) => Entry.Key.CompareTo(Key) == 0)].Value;
            }
            set
            {
                Entries[Entries.FindIndex((LibrarySubjectEntry<TKey, TVal> Entry) => Entry.Key.CompareTo(Key) == 0)].Value = value;
            }
        }
        public TSubKey SubjectKey;
        public List<LibrarySubjectEntry<TKey, TVal>> Entries;
    }
    public class LibrarySubjectEntry<TKey, TVal>
        where TKey : IComparable
    {
        public LibrarySubjectEntry(TKey Key, TVal Value)
        {
            this.Key = Key;
            this.Value = Value;
        }
        public TKey Key;
        public TVal Value;
    }
}