Entity NBT Blocks Not Working?

In my mod, I am making it so if you kill a Charged Creeper, it will drop a special item.

For this, I need to use the "Powered" NBT tag of a Creeper, to determine if it is charged or not.

I tried using the "get entity NBT" block, but it would never work.

Could someone help me figure out why it isn't working? Or is there a different way to do it, maybe with commands?

Any help would be greatly appreciated!