It doesn't work on zombies because you can't single them out like you do humans - you just see a pile of zombies, possibly with someome you know amongst them. It's one of the big advantages of the zombie hordes, that people can't target the weak ones tho whittle down their numbers. The skill does carry over if you get eaten though, so as a zombie you can single out weak Harmanz.
This . Also useful are First Aid (easy XP), and better axe skills. Headshot (when you get there) is considered rude by the zombies and will get you eaten first.