How to add to navigation property? (newbe q)

Entities cannot be added to a RelatedEntityList belonging to a detached entity

   Hi,
I'm new to Devforce, and trying to build my first app. What I'm trying is:
A domain model with two entities, Order and OrderDetail.
Order has an OrderDetails navigation property
OrderDetails does NOT have a navigation property to Order (I don't want all of my entities point to everything else)

My code looks like this:

var em = new DomainModel.DomainModelEntityManager(true);

var d = em.CreateEntity<DomainModel.OrderDetail>();
// set detail properties (...)
d.EntityAspect.AddToManager();

var o = em.CreateEntity<DomainModel.Order>();
// set order properties (...)
o.OrderDetails.Add(d);
o.EntityAspect.AddToManager();

em.SaveAsync(savedcallbackhandler, null);

I'm getting a NullReferenceException on o.OrderDetails.Add(d)

What am I doing wrong?

TIA,

Patrick