Упрямый объект не будет сериализоваться с помощью protobuf-net

Я интегрирую protobuf-net в наше решение на основе служб WCF, но столкнулся с проблемой, которую не могу понять. Следующий класс будет сериализоваться нормально, все, кроме свойства ObjectId.

/// <summary>
/// A service data object that represents a user of the system.
/// </summary>
[DataContract(Namespace = "http://LINKS.Service.Security.Administration", Name = "User")]
public sealed class UserMessagePart : IMessagePart
{
    private ObjectId objectId;
    private string userName;
    private string firstName;
    private string lastName;
    private string middleName;
    private string gender;
    private string emailAddress;
    private bool isAccountDisabled;
    private string disabledMeaning;
    private DateTime createDtTm;
    private DateTime disabledDtTm;
    private VersionMessagePart version;

    /// <summary>
    /// Main constructor.
    /// </summary>
    public UserMessagePart(ObjectId objectId, string userName, string firstName, string lastName, string middleName,
                    string gender, string emailAddress, bool isAccountDisabled, string disabledMeaning, DateTime createDtTm,
                    DateTime disabledDtTm, VersionMessagePart version)
    {
        this.objectId = objectId;
        this.userName = userName;
        this.firstName = firstName;
        this.lastName = lastName;
        this.middleName = middleName;
        this.gender = gender;
        this.emailAddress = emailAddress;
        this.isAccountDisabled = isAccountDisabled;
        this.disabledMeaning = disabledMeaning;
        this.createDtTm = createDtTm;
        this.disabledDtTm = disabledDtTm;
        this.version = version;
    }

    /// <summary>
    /// Parameterless constructor.
    /// </summary>
    public UserMessagePart(){}

    /// <summary>
    /// The unique identifier for this user.  
    /// </summary>
    [DataMemberAttribute(IsRequired = true, Name = "ObjectId", Order = 0)]
    public ObjectId ObjectId
    {
        get { return objectId; }
        set { objectId = value; }
    }

    /// <summary>
    /// The user's login identity.
    /// </summary>
    [DataMemberAttribute(IsRequired = true, Name = "UserName", Order = 1)]
    public string UserName
    {
        get { return userName; }
        set { userName = value; }
    }

    // ...abbreviated code for readability...

    /// <summary>
    /// Version information for this user
    /// </summary>
    [DataMemberAttribute(IsRequired = true, Name = "Version", Order = 11)]
    public VersionMessagePart Version
    {
        get { return version; }
        set { version = value; }
    }
}

Вот класс, который играет со мной:

/// <summary>
/// Uniquely identifies any <see cref="IMessagePart"/> implementation in the system.
/// </summary>
[DataContract(Namespace = "http://LINKS.Service.Core", Name = "ObjectIdentifier")]
public class ObjectId
{
    private string id;
    private string domain;
    private string modelName;
    private long instanceId;
    private int versionNumber;

    /// <summary>
    /// Default constructor. (required for Protobuf-net)
    /// </summary>
    public ObjectId()
    {
    }

    /// <summary>
    /// Main constructor.
    /// </summary>
    public ObjectId(string domain, string modelName, long instanceId, int versionNumber)
    {
        id = string.Format("{0}#{1}#{2}#{3}", domain, modelName, instanceId, versionNumber);
        this.domain = domain;
        this.modelName = modelName;
        this.instanceId = instanceId;
        this.versionNumber = versionNumber;
    }

    /// <summary>
    /// The unique identifier for the <see cref="ObjectId"/>.  The format of this string is not
    /// guaranteed and should only be treated as a unique key.  No attempts to parse it should be 
    /// made by client applications.
    /// </summary>
    [DataMemberAttribute(IsRequired = true, Name = "Id", Order = 0)]
    public string Id
    {
        get { return id; }
        set
        {
            id = value;
            string[] parts = id.Split('#');
            domain = parts[0];
            modelName = parts[1];
            instanceId = long.Parse(parts[2]);
            versionNumber = int.Parse(parts[3]);
        }
    }

    /// <summary>
    /// The system domain that the <see cref="ObjectId"/> originated from.
    /// </summary>
    public string Domain
    {
        get { return domain; }
    }

    /// <summary>
    /// The type of object that this <see cref="ObjectId"/> refers to.
    /// </summary>
    public string ModelName
    {
        get { return modelName; }
    }

    /// <summary>
    /// The object instance identifier for the object that this <see cref="ObjectId"/> 
    /// refers to.  Typically, this is a database key.
    /// </summary>
    public long InstanceId
    {
        get { return instanceId; }
    }

    /// <summary>
    /// The version instance of the object referred to by this <see cref="ObjectId"/>
    /// </summary>
    public int VersionNumber
    {
        get { return versionNumber; }
    }

    /// <summary>
    /// Returns a string representation of the object identifier.
    /// </summary>
    new public string ToString()
    {
        return id;
    }
}

Я пробовал несколько вещей без везения. Любые мысли будут очень признательны!