How do I declare a C# property which uses a lambda return method in ProtoBuf?

I have a C# class which I am making into a ProtoBuf class. The class is structured as followed:

[ProtoContract]
public class TestClass : TestBaseClass
{
     [ProtoMember(1)]
     public int Age {get; set};

     [ProtoMember(2)]
     public string Name {get; set;}

     [ProtoMember(3)]
     public AnimalClass Animal {get; set; }

     [ProtoMember(4)]
     protected internal override long? AnimalId => Animal?.TestAnimalId;
}

I am trying to Serialize this class using the ProtoBuf.Serializer.Serialize() function. This is done in the following class:

        public byte[] TestSerialize(object obj)
        {
            using (var memory = new MemoryStream())
            {
                ProtoBuf.Serializer.Serialize(memory, obj);
                return memory.ToArray();
            }
        }

However, I am receiving a 'System.InvalidOperationException' during serialization, stating that it cannot make the change to 'AnimalId'. How would I serialize this class to a byte array?

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