(solved and intended)Custom Spawner with "MaxNearbyEntities" freezes

I was playing around with custom spawners but when I used the tag "MaxNearbyEntities" , the spawner froze and nothing happened...

What I expected to happen was...
... the spawner would just work as normal. It would not freeze and it would stop spawning mobs if there was already a certain amount of mobs around the spawner. (Just what the tag does, or should do)

What actually happened was...
...the spawner froze and did absolutely nothing.

How to reproduce:
1.Give yourself a command block and place it down.
2. Use the setblock command to setblock a custom mob spawner with the "MaxNearbyEntities" -tag.

/setblock <x> <y> <z> minecraft:mob_spawner 0 replace

{EntityId:Zombie,MaxNearbyEntities:2}

3. Power the command block and notice that the little "mob" the spawner inside freezes, there are no particles and that is does nothing.