Command Block /execute at all UNnamed mobs of same type

I have various named Zombies. I want to do something with all OTHER Zombies.

/execute @e[name=Zombie] ~ ~ ~ summon ArmorStand ~ ~ ~

Above does not seem to work. Any way to target Zombies that have no CustomName?

1.13 Update

You can use the unless subcommand of /execute to execute as all zombies that don't have the custom name:

execute
  as @e[type=zombie]
  unless data entity @s CustomName
run ...