MessagePack erroring

            var sendData = new SocketSendData(new InitialConnectionData(Helpers.GetPCDisplayName(),
                Helpers.GetIPAddress(), Helpers.GetOSVersion()));
            var buf = MessagePackSerializer.Serialize(sendData);
            ClientSocket.Send(buf, 0, buf.Length, SocketFlags.None);

[MessagePackObject]
    public class InitialConnectionData
    {
        [SerializationConstructor]
        public InitialConnectionData(string pcName, string dnsIpAddr, string osVersion)
        {
            PCName = pcName;
            DnsIpAddr = dnsIpAddr;
            OSVersion = osVersion;
        }

        [Key(0)] public string PCName { get; set; }

        [Key(1)] public string MachineIPAddr { get; set; }

        [IgnoreMember] public string DnsIpAddr { get; set; }

        [IgnoreMember] public Socket LinkedSocket { get; set; }

        [Key(3)] public string OSVersion { get; set; }