This is a pretty silly rule; there are lots of hulking constructs out there (e.g. most golems) who have no Con at all.
Don't forget undead. Zombie have no Con.
Also, wouldn't this mean that all ghosts are fully invisible? Since your body mass is based off of Str and Con. Any combination of -- and -- is going to result in --.
I follow a general rule: better to ask and be told no than not to ask at all.