MongoDB: Deserializing complex object returns NULL

Using C# Mongo DB Driver version, 2.4.4 and I try to deserialize a complex class. It has been stored correctly in the MongoDB, but when being deserialized it is null.

This is my datamodel:

public abstract class Document
{
    public DocumentId Id { get; private set; }

    public DocumentProperty TestProperty { get; private set; }

    protected Document() { }

    public void SetId(string value)
    {
        if (string.IsNullOrEmpty(value)) throw new ArgumentNullException("value");
        Id = new DocumentId(value);
    }

    public void AddProperty(DocumentProperty property)
    {
        if (property == null) throw new ArgumentNullException("property");
        TestProperty = property;
    }
}

public class DocumentId
{
    public string Value { get; }

    public DocumentId(string value)
    {
        Value = value;
    }
}

public abstract class DocumentProperty
{
    public string Value { get; }

    protected DocumentProperty()
    {
    }

    protected DocumentProperty(string value)
    {
        Value = value;
    }
}

public class TitleDocumentProperty : DocumentProperty
{
    public TitleDocumentProperty() { }

    public TitleDocumentProperty(string value) : base(value)
    {
    }
}

This is my mapping code:

public class MongoClassMapper
{
    public static void InitializeMap()
    {
        BsonClassMap.RegisterClassMap<DocumentProperty>(map =>
        {
            map.MapProperty(property => property.Value).SetDefaultValue("123");
            map.SetIsRootClass(true);
        });

        BsonClassMap.RegisterClassMap<TitleDocumentProperty>(map =>
        {
        });

        BsonClassMap.RegisterClassMap<Document>(map =>
        {
            map.MapProperty(document => document.Id);
            map.MapProperty(document => document.TestProperty);
            map.SetIsRootClass(true);
        });

    }
}

This is my methods for adding and retrieving data from Mongo:

public async Task<string> Add(Document document)
    {
        await _documents.InsertOneAsync(document);
        return document.Id.Value;
    }

    public async Task<Document> Get(DocumentId id)
    {
        var mongoDocuments = await _documents.Find(document => document.Id == id)
            .ToListAsync();

        return mongoDocuments.SingleOrDefault();
    }

This is the code that I use for testing:

private static MongoCache _cache;

    static void Main(string[] args)
    {
        MongoClassMapper.InitializeMap();
        _cache = new MongoCache(ConfigurationManager.ConnectionStrings["connName"].ConnectionString, "dbName");
        string id = ItShouldCreateRecord().Result;
        var doc = ItShouldGetRecord(id).Result;
    }

    private static Task<string> ItShouldCreateRecord()
    {
        //Arrange
        Document document = new FakeDocument();
        document.SetId(Guid.NewGuid().ToString());
        document.AddProperty(new TitleDocumentProperty("this is a title"));

        //Act
        string id = _cache.Add(document).Result;

        //Assert
        //Assert.NotEmpty(id);
        return Task.FromResult(id);
    }

    private static Task<Document> ItShouldGetRecord(string id)
    {
        //Arrange

        //Act
        return _cache.Get(new DocumentId(id));

        //Assert
        //Assert.NotNull(doc);
    }
}

[BsonIgnoreExtraElements]
public class FakeDocument : Document
{
}

I expected that when I retrieve the document (using _cache.Get()) from the DB, that the property TestProperty would have an actual value. Currently, it is NULL.