Nothing's bugged about the helmet its just you're missing armature groups (could be simple as re-naming, probably neck , clavicle or spine# f I had to guess ex Neck1 vs just Neck) on the zombie's skeleton. Easiest way is to just rig it bypassing those groups (ex add them to groups the zombie skeleton already has), alternative way is to add the groups of interest to the skeleton, but that's more involved.