mongodb c# driver - inheritance, mappings and serialization issue

I have following class hierarchy for object stored in mongodb (I store only Branch objects and Entities in their graph)

public class Branch : Aggregate { public IEnumerable<LocalizableText> Description { get; set; } public ObjectId PartnerId { get; set; } public double Latitude { get; set; } public double Longitude { get; set; } public string Timetable { get; set; } public IEnumerable<Discount> Discounts { get; set; } public IEnumerable<Category> Categories { get; set; } public IEnumerable<Phone> Phones { get; set; } public byte[] Icon { get; set; } public byte[] Image { get; set; } } public abstract class Aggregate : Entity { public ObjectId Id { get; set; } } public abstract class Entity { public IEnumerable<LocalizableText> Name { get; set; } } 

I have the following registrations running at server start for this hierarchy:

 BsonClassMap.RegisterClassMap<Entity>(); BsonClassMap.RegisterClassMap<Aggregate>(cm => { cm.AutoMap(); cm.SetIdMember(cm.GetMemberMap(a => a.Id)); }); BsonClassMap.RegisterClassMap<Branch>(); 

But when I'm running this query

return await Collection.Aggregate().Match(x => x.PartnerId == partnerId) .Group(x => x.PartnerId, g => new { PartnerId = g.Key, g.First(x => x.Name != null).Name, Description = g.First(x => x.Id == branchId).Name, g.First(x => x.Id == branchId).Discounts, Id = branchId }) .Project(g => new Branch() { Id = g.Id, Name = g.Name, Description = g.Description, Discounts = g.Discounts, PartnerId = g.PartnerId }).FirstOrDefaultAsync(); 

I'm getting the following exception:

Test method ShouldGetBranchToolTipAsync threw exception:

System.ArgumentOutOfRangeException: The memberInfo argument must be for class Branch, but was for class Aggregate.

Parameter name: memberInfo at MongoDB.Bson.Serialization.BsonClassMap.EnsureMemberInfoIsForThisClass(MemberInfo memberInfo) at MongoDB.Bson.Serialization.BsonClassMap.MapMember(MemberInfo memberInfo) at MongoDB.Driver.Linq.Translators.AggregateProjectionTranslator.SerializerBuilder.BuildProjectedSerializer(ProjectionMapping mapping) at MongoDB.Driver.Linq.Translators.AggregateProjectionTranslator.SerializerBuilder.BuildMemberInit(MemberInitExpression node) at MongoDB.Driver.Linq.Translators.AggregateProjectionTranslator.SerializerBuilder.Build(Expression node) at MongoDB.Driver.Linq.Translators.AggregateProjectionTranslator.SerializerBuilder.Build(Expression node, IBsonSerializerRegistry serializerRegistry) at MongoDB.Driver.Linq.Translators.AggregateProjectionTranslator.TranslateProject(Expression1 projector, IBsonSerializer1 parameterSerializer, IBsonSerializerRegistry serializerRegistry) at MongoDB.Driver.IAggregateFluentExtensions.ProjectExpressionProjection2.Render(IBsonSerializer1 documentSerializer, IBsonSerializerRegistry serializerRegistry)

What is the cause of this? Are mapping incorrect or called at a wrong time?