I just received a used Pentax 67 body that I purchased off ebay. You're supposed to be able to fire the body without film in it. I downloaded the manual off the internet and tried firing the shutter with the back door open and the film advance would not advance and the shutter would not fire. I started to worry. But then I loaded up a roll of film and the MLU worked, and the shutter fired. Should I be worried that the camera will not fire without film like it's supposed to? Or am I doing something wrong trying to do so?