SaveAsync() am I missing something?

So I've been trying to implement the new SaveAsync() methods in 4.5.6. There is not much info or guidance on how these should be used, or not that I can find anyway.

I always get an error "DataPortal.Update failed (TemplateEdit.IsBusy == true)"

On debugging the DataPortal Update is marking the object as busy and then bailing before it ends.

public async Task<Model> Post(Modelmodel)        {

            // do something to create and populate an object            ...            obj = await obj.SaveAsync();            return new Model(obj);
        }

Its as simple as that. DataPortal_XYZ are the regular ol' voids etc. Is this correct?

There are no business rules so I've ruled (pardon the pun) them out.