InvalidOperationException when using updatemodel with EF4.3.1

When I update my model I get an error on a child relation which I also try to update.

My model, say Order has a releationship with OrderItem. In my view I have the details of the order together with an editortemplate for the orderitems. When I update the data the link to Order is null but the orderid is filled, so it should be able to link it, TryUpdateModel returns true, the save however fails with:

InvalidOperationException: The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.]

My update method:

    public ActionResult ChangeOrder(Order model)
    {
        var order = this.orderRepository.GetOrder(model.OrderId);

        if (ModelState.IsValid)
        {
            var success = this.TryUpdateModel(order);
        }

        this.orderRepository.Save();

        return this.View(order);
    }

I tried all solutions I saw on SO and other sources, none succeeded.

I use .Net MVC 3, EF 4.3.1 together with DBContext.