Map a nullable entity

I am trying to map an id to object with AutoMapper and I have been trying to figure this out for two days. Any help would be appreciated.

Here are my domain objects:

public class Entity
{
    public virtual Guid Id { get; protected set; }
}

public class Customer : Entity
{
    public virtual User SalesPerson { get; set; }
}

public class User : Entity
{
    public virtual string Email { get; set; }
}

Here is the edit model I'm using. Sales Person is a drop-down list of users with a "None" option (value="") as a customer may not have a sales person.

public class Cu开发者_如何学GostomerEditModel
{
    [DisplayName("Sales Person")]
    public Guid? SalesPerson { get; set; }
}

And here is where I am stuck. These are the mappings I have tried so far to map the edit model back to domain model:

cfg.CreateMap<CustomerEditModel, Customer>()
    .ForMember(d => d.SalesPerson, o => o.ResolveUsing<EntityResolver<User>>().FromMember(s => s.SalesPerson));

cfg.CreateMap<CustomerEditModel, Customer>()
    .ForMember(d => d.SalesPerson, o => o.MapFrom(s => s.SalesPerson.HasValue ? s.SalesPerson.Value : null);

cfg.CreateMap<CustomerEditModel, Customer>()
    .ForMember(d => d.SalesPerson, o => o.MapFrom(s => s.SalesPerson != null ? s.SalesPerson : null));

public class EntityResolver<T> : ValueResolver<Guid, T>
    where T : Entity
{
    protected override T ResolveCore(Guid source)
    {
        return session.Get<T>(source, LockMode.None);
    }
}

I made the following changes and things appear to be working as expected.

public class EntityResolver<id, entity> : ValueResolver<id, entity>
    where entity : Entity
{
    protected override entity ResolveCore(id source)
    {
        if (source != null)
            return session.Get<entity>(source, LockMode.None);
        else
            return null;
    }
}

...

cfg.CreateMap<CustomerEditModel, Customer>()
    .ForMember(d => d.SalesPerson, o => o.ResolveUsing<EntityResolver<Guid?, User>>().FromMember(s => s.SalesPerson));