Entity Framework Core 2 querying multi level collections with theniclude

I created two classes:

public class A {
  public int Id { get; set; }
  public ICollection<B> Bs { get; set; }
}

public class B {
  public ICollection<C> C1s { get; set; }
  public ICollection<C> C2s { get; set; }
}

then I tried to fetch them with ThenInclude method:

var result = context.As          //public DbSet<A> As { get; set; }
  .Include(a => a.Bs)
    .ThenInclude(b => b.C1s)
  .Include(a => a.Bs)
    .ThenInclude(b => b.C2s)
  .SingleOrDefaultAsync(a => a.Id.Equals(id)); //id is given

return await result;

But unfortunately both C1s and C2s collections are empty.

How to retrieve C entities which are related to B one?