I want to make Nether Star drops from the WitherBoss more rare. I am setting it to a 15% drop rate, but no matter what it's still 100%.

My WitherBoss config is set to:
  WitherBoss:
    Coin_Minimum: 20.0
    Coin_Maximum: 30.0
    Coin_Percent: 100.0
    Drops:
    - nether star:1-1:15

I'm using ecoCreature-0.2.2-SNAPSHOT.jar and CB 1.4.6-R0.1.

FYI I've also tried using "NETHER_STAR" and "399"