Testfor a mobs health doesn't work ?

So I'm trying to detect the health of a ghast, but it ain't working? I summon in the ghast using: {Health:500,Attributes:[{Name:generic.maxHealth,Base:500.0d}]}

But when I do the /testfor @e[type=Ghast] {Health:500s} it doesn't work?

"[16:03:55] Ghast did not match the required data structure"

Am I missing out on some change, or maybe I'm just dumb.