X100 in Aperture Priority...

OK, here's my latest X100 befuddlement:

I'm experimenting with Aperture Priority and have the camera set at f5.6, ISO 400 and "A" on the shutter dial. The X100 will not allow the shutter to drop below 1/30, no matter how dark the scene is. When I point the camera at a dark area in my office, I get a nice looking histogram, the big"A" in red, and the shutter speed, aperture and ISO displayed along the bottom. When I half-press the shutter, I get AF and the "30" turns green, as if it's a proper exposure. It isn't, of course - the resulting picture is way dark.

I don't have auto ISO turned on, but I changed the min. shutter speed setting in auto ISO down to 1/4 from where it had been at 1/30, in case that was the problem. It wasn't.

Yes, I looked in the manual and found nothing. Any ideas why Aperture Priority only works from 1/30 and faster on my X100? I really hope this isn't how it's supposed to work.

Thanks,

Gary