Pentax K10D won't work with Pentax K100 len...

I just bought a Pentax K10D which shoots very good photos. I also have an old Pentax K100 film camera. So I was hoping that I could use the lens from the old camera on the new one. The lens seems to screw on correctly, but the shutter won't work and I can't take a picture. I changed all the settings to M but still nothing. Do I need an adapter to use my old lenses on the Pentax K10D?
Thanks