Embedded custom type deserializes to BsonValue, not the the custom type

[cmharris]

I have the following custom mappers, where the type of WorldItem.Position is a Vector3. When deserializing WorldItem, the Vector3 type is not deserialized to the type and is instead a BsonValue. How do I embed custom types?

BsonMapper.Global.RegisterType<WorldItem>(
   serialize: w =>
   {
      var doc = new BsonDocument
      {
         ["position"] = new BsonValue(w.Position),
         ["rotation"] = new BsonValue(w.Rotation)
      };
      return doc;
   },
   deserialize: doc => new WorldItem(doc["_id"], doc["position"], doc["rotation"])
);

BsonMapper.Global.RegisterType(
   serialize: v => new BsonArray(v.X, v.Y, v.Z),
   deserialize: b => new Vector3(b[0].AsDouble, b[1].AsDouble, b[2].AsDouble)
);