JSON.NET Serializes Empty JSON

I am using MetadataType to define Json.NET attributes for the following type, then serializing it using Json.NET inside its ToString() method:

namespace ConsoleApp1
{
    public interface ICell
    {
        int Id { get; }
    }
    public interface IEukaryote
    {
        System.Collections.Generic.IEnumerable<ICell> Cells { get; }
        string GenericName { get; }
    }
    public sealed partial class PlantCell
        : ICell
    {
        public int Id => 12324;
    }
    public sealed partial class Plant
        : IEukaryote
    {
        private readonly System.Collections.Generic.IDictionary<string, object> _valuesDict;
        public Plant()
        {
            _valuesDict = new System.Collections.Generic.Dictionary<string, object>();
            var cells = new System.Collections.Generic.List<PlantCell>();
            cells.Add(new PlantCell());
            _valuesDict["Cells"] = cells;
            _valuesDict["GenericName"] = "HousePlant";
        }
        public System.Collections.Generic.IEnumerable<ICell> Cells => _valuesDict["Cells"] as System.Collections.Generic.IEnumerable<ICell>;
        public string GenericName => _valuesDict["GenericName"] as string;
        public int SomethingIDoNotWantSerialized => 99999;
        public override string ToString()
        {
            return Newtonsoft.Json.JsonConvert.SerializeObject(this,
                new Newtonsoft.Json.JsonSerializerSettings()
                {
                    ContractResolver = new Newtonsoft.Json.Serialization.CamelCasePropertyNamesContractResolver()
                }
            );
        }
    }
    [System.ComponentModel.DataAnnotations.MetadataType(typeof(PlantMetadata))]
    public sealed partial class Plant
    {
        [Newtonsoft.Json.JsonObject(Newtonsoft.Json.MemberSerialization.OptIn)]
        internal sealed class PlantMetadata
        {
            [Newtonsoft.Json.JsonProperty]
            public System.Collections.Generic.IEnumerable<ICell> Cells;
            [Newtonsoft.Json.JsonProperty]
            public string GenericName;
            //...
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            var plant = new Plant();
            System.Console.WriteLine(System.String.Format("Output is {0}", plant.ToString()));
            System.Console.ReadKey();
        }
    }
}

My problem is that Plant.ToString() will return '{}'. Why is that? It was working before. The only change I made was in PlantMetadata where I altered the MemberSerialization to OptIn instead of OptOut, as I had less properties I wanted included than left out.