I would agree with Moke123 that just setting the default property to something ought to be simpler. However, if it's not working for you on a combo on a form but it is for other controls, that's an oddity. The fact that it's not working in a particular instance makes me think that there's a subform or navigation form involved. If there is, then neither method is going to work if the reference isn't correct.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.