Timed Access Control Set on AEBS and AE stops working?

Hi

I have an Airport Extreme and recently purchased a new Airport Express to extend my network and use AirPlay.

I have reset my Airport Extreme as I want to set up new network names, passwords and also use the Timed Access Control. This is timed for the children, but also to stop others trying to hack into my networks.

When the Wireless Clients - Unlimited (default) is left unchanged, the Airport Express shows in the main Airport Utility WIndow. When I add a few clients to the list with their Mac addresses and change the Unlimited settings to No Access, the Airport Express stops working.

Is there anyway around this, or do I need to set up the AEBS fully before adding the AE?

Thanks in advance

Matt