Mob spawners with a Weight tag crash the game.

When I create a mob spawner with a "Weight" tag in SpawnPotentials, the game crashes and I can no longer reload the world without it crashing again. Here is an example command which would crash the game:

/setblock ~ ~ ~ minecraft:mob_spawner 0 replace {SpawnData:

{id:sheep}

,SpawnPotentials:[{Entity:

{id:sheep}

,Weight:0.6},{Entity:

{id:cow}

,Weight:0.4}]}