How to get old analog flashes to work on digital cameras

I have an old Sunpak auto 433 D Thyristor flash, and I wanted to use it on my Nikon D50 DSLR, but when I attached it to the hot shoe, my camera refused to take a picture. I figured out that if you put a small piece of paper over the bottom two contacts of the flash, and slide the flash into the hot shoe with the paper wedged in between the flash and the hotshoe, the flash works!