Error in entity framework update with automapper. The role '...' of the relationship '...' has multiplicity 1 or 0..1

Question

I'm trying to update a model that is getted by automapper bind.

When my update function execute

Context.Entry(obj).State = EntityState.Modified;

entity framework throw this exception

Multiplicity constraint violated. The role 'LegislationCategory_Legislations_Source' of the relationship 'Creci.Infra.Data.Context.LegislationCategory_Legislations' has multiplicity 1 or 0..1.

If i try to update the model without use automapper it's works fine.

The relations is

public class LegislationCategoryMap : EntityTypeConfiguration<LegislationCategory>
    {
        public LegislationCategoryMap()
        {
            ...

            HasMany(x => x.Legislations)
                .WithRequired(x => x.Category)
                .WillCascadeOnDelete(true);
        }
    }

public class LegislationMap : EntityTypeConfiguration<Legislation>
    {
        public LegislationMap()
        {
            ...

            HasRequired(x => x.Category);
        }
    }

And my mapper profile configuration is

CreateMap<EditLegislationViewModel, Legislation>()
                .EqualityComparision((vm, m) => vm.Id == m.Id)
                .AfterMap((vm, m) =>
                {
                   ...

                    if (vm.Category == null && vm.CategoryId != 0)
                    {
                        m.Category = new LegislationCategoryRepository().GetById(vm.CategoryId);
                    }
                });

I used aftermap to map category to Legislation object when viewmodel is binded from request.

my controller code is

var model = Mapper.Map(viewModel, repository.GetById(id));
            repository.Update(model);

and the update function

public void Update(TEntity obj)
        {
            Validate(obj);

            using (var transaction = Context.Database.BeginTransaction())
            {
                try
                {
                    Context.Entry(obj).State = EntityState.Modified;
                    Context.SaveChanges();
                    transaction.Commit();
                }
                catch (Exception)
                {
                    transaction.Rollback();
                    throw;
                }
            }
        }