entitydata doesn't work when name is +/-

I tried to change the Name of an WitherSkull using Entitydata.

/entitydata @e[type=WitherSkull,name=+] {CustomName:-}

This Doesn't Work, when the Skull is named "+", this:

/entitydata @e[type=WitherSkull,name=Add] {CustomName:Remove}

does, when the Skull is named Add