When searching for a skeleton with "{SkeletonType:0}" or Wither Skeleton with "{SkeletonType:1}" "{SkeletonType:}" doesn't work

When I want to do something like: /scoreboard players tag @e[type=Skeleton] add WitherSkeleton

{SkeletonType:1}

just says that Wither Skeleton's and commands tags don't match, but if I want to summon him using "

{SkeletonType:1}

" it works. If this is intended please tell me how can I separate Wither Skeletons and Skeletons without using that tag.