http://stackoverflow.com/questions/30730007/mongodb-c-sharp-error-on-first-push/30732823

gistfile1.cs

using MongoDB.Bson;
using MongoDB.Driver;
using MongoDB.Bson.Serialization.Attributes;

class Entity
{
	public ObjectId Id { get; set; }
	public string Prop1 { get; set; }

	public List<EmbeddedEntity> Embeds { get; set; }

	public Entity()
	{
		Embeds = new List<EmbeddedEntity>();	
	}
}

class EmbeddedEntity
{
	public string Prop2 { get; set; }
}

var client = new MongoClient();
var db = client.GetDatabase("test");
var col = db.GetCollection<Entity>("entities");

db.DropCollectionAsync("entities").Wait();

var entity = new Entity { Prop1 = "Yes!" };

col.InsertOneAsync(entity).Wait();

var update = Builders<Entity>.Update.Push(x => x.Embeds, new EmbeddedEntity { Prop2 = "No!!!" });

col.UpdateOneAsync(x => x.Id == entity.Id, update).Wait();