Error saving to bank

I would like some help.

When I try to save the registry, the following message appears:

  

23502: null value in column "id" violates not-null constraint

The primary keys are not autoincrement, I'm using a function that generate the primary keys for me. My code that I'm doing to save:

 public async Task Salvar(T obj)
    {
        Db.Set<T>().Add(obj);
        await Db.SaveChangesAsync();//TA DANDO ERRO AQUI
    }

My id is already going with a value.