Use unsigned version format. This way can use one additional bit, and when encountering invalid revlogs with the first bit set don't produce python warnings or strange error messages.

#!/bin/sh
# http://www.selenic.com/mercurial/bts/issue433

hg init a
cd a       
echo a > a 
hg commit -Ama 

hg parents -r 0 doesnotexist
true