New Nikon 17-55 f/2.8: Defective or Normal?

Hi,
I would like to understand whether this is expected, or I have a defective lens:
Brand new Nikon 17-55 f/2.8 on a Nikon D50.

I cannot control the f/ stop in any mode except A mode. i.e. if I am in P mode or Auto mode, etc. the F stop remains at 2.8 regardless of conditions. It would not go above that. Please assist. Thanks.