Hostile entities not spawning in custom biomes

I made a custom biome, but no hostile entities were spawning, despite of me putting it into the spawn list. I checked the code and I saw the line "this.addSpawn(EntityClassification.CREATURE, new Biome.SpawnListEntry(EntityType.ZOMBIE, 15, 1, 5));", and just out of curiosity I changed "CREATURE" to "MONSTER", and it worked! This is very annoying, and it took me 1 hour to figure it out. PLEASE FIX

Also I cannot find the code for "Entity.Mob" when it's in the spawn list, so I don't even know if it does anything.. And if it doesn't do anything it should allow the spawning of hostile entities.