Update Embedded document in mongodb using C#

Suppose you have the next class. It contains the systems in which the agent has worked

public class AgentHistory
{
    public ObjectId Id { get; set; }
    public Guid SystemId { get; set; }
    public Guid CampaignId { get; set; }
    public List<Agent> Agents { get; set; }
}

Now when I get a new agent I do the next thing:

 public override AgentHistory Save(AgentHistory agent)
    {
        if (agent == null)
            throw new ArgumentNullException("agent");
        if (_repository.Exists(agent))
        {
            AgentHistory dbEntity = _repository.FindById(agent.SystemId, agent.CampaignId);
            dbEntity.Agents.AddRange(agent.Agents);
            _repository.UpdateAgentHistory(dbEntity);
        }
        else
        {
            _repository.Save(agent);
        }
        return agent;
    }

And the next method in repository:

public void UpdateAgentHistory(AgentHistory updatedEntity)
    {
        QueryComplete query = Query.EQ("_id", BsonValue.Create(updatedEntity.Id));

        MongoCollection.Update(query, Update.Set("Agents", BsonArray.Create(updatedEntity.Agents)), UpdateFlags.None, SafeMode.True );
    }

I get the next exception .NET type Riverdale.Domain.BO.Agent cannot be mapped to a BsonValue. What am I doing wrong? What is the right approach to updating embedded collections?

Here is a simpler console app which throws(just as a demo):

  public class Agent
    {
        [BsonId]
        public string LocalIdentifier { get; set; }

        public string AgentName { get; set; }
    }

    public class A
    {
        public ObjectId Id { get; set; }
        public Guid SystemId { get; set; }
        public Guid CampaignId { get; set; }
        public Agent[] Agents { get; set; }
    }

    public class AgentHistoryRepository
    {
        public bool Exists(A agentHistory)
        {
            return _mongoCollection.FindOne(BuildIdentityQuery(agentHistory)) != null;
        }

        public void Delete(A agentHistory)
        {
            _mongoCollection.Remove(BuildIdentityQuery(agentHistory));
        }

        public List<string> GetAgentsForASystem(Guid systemGuid)
        {
            QueryComplete query = Query.EQ("SystemId", systemGuid);
            return _mongoCollection.Find(query).SelectMany(x => x.Agents.Select(z => z.AgentName)).Distinct().ToList();
        }

        public List<string> GetAgentsForACampaign(Guid systemGuid, Guid campaignGuid)
        {
            QueryComplete query = Query.EQ("CampaignId", campaignGuid);
            if (systemGuid != Guid.Empty)
                query = Query.And(new[] {query, Query.EQ("SystemId", systemGuid)});
            return _mongoCollection.Find(query).SelectMany(x => x.Agents.Select(z => z.AgentName)).Distinct().ToList();
        }

        public AgentHistoryRepository()
        {
            string connectionString = "mongodb://localhost/Sample";
            var mgsb = new MongoUrlBuilder(connectionString);
            var MongoServer = MongoDB.Driver.MongoServer.Create(mgsb.ToMongoUrl());
            var MongoDatabase = MongoServer.GetDatabase(mgsb.DatabaseName);
            _mongoCollection = MongoDatabase.GetCollection<A>("AgentHistory");
        }


        private MongoCollection<A> _mongoCollection;

        private QueryComplete BuildIdentityQuery(A agentHistory)
        {
            QueryComplete query = Query.And(Query.EQ("SystemId", agentHistory.SystemId),
                                            Query.EQ("CampaignId", agentHistory.CampaignId));
            return query;
        }

    public void Save(A entity)
        {
            _mongoCollection.Insert(entity, SafeMode.True);
        }

        public void UpdateAgents(A entity)
        {
            _mongoCollection.Update(BuildIdentityQuery(entity), Update.Set("Agents", entity.Agents.ToBsonDocument()));
        }
    }

    internal class Program
    {
        public static void Main()
        {

            var objectToSave =  new A {Id = ObjectId.GenerateNewId(), CampaignId=Guid.NewGuid(), SystemId =Guid.NewGuid() ,
                                     Agents = new [] {new Agent{LocalIdentifier="agent", AgentName= "name"}}};

            var repo = new AgentHistoryRepository();
            repo.UpdateAgents(objectToSave);
            objectToSave.Agents = new[] { new Agent { LocalIdentifier = "agent2", AgentName = "name2" } };
            repo.UpdateAgents(objectToSave);
            var objectToSave2 = new A
            {
                Id = ObjectId.GenerateNewId(),
                CampaignId = Guid.NewGuid(),
                SystemId = objectToSave.SystemId,
                Agents = new [] { new Agent { LocalIdentifier = "agent", AgentName = "name" } }
            };
            repo.UpdateAgents(objectToSave2);
            foreach (var agentName in repo.GetAgentsForASystem(objectToSave.SystemId))
                Console.WriteLine(agentName);
        }
    }