Issue with volcanoes and biomes

[Garagoth]

I periodically get this exception:

2014-08-03 21:19:38 [INFO] [STDOUT] WorkerUpdateChunks got an exceptionjava.lang.ArrayIndexOutOfBoundsException: -1 
2014-08-03 21:19:38 [INFO] [STDERR] java.lang.ArrayIndexOutOfBoundsException: -1 
2014-08-03 21:19:38 [INFO] [STDERR] at net.minecraft.world.biome.WorldChunkManager.func_76931_a(WorldChunkManager.java:194) 
2014-08-03 21:19:38 [INFO] [STDERR] at net.minecraft.world.biome.BiomeCacheBlock.<init>(SourceFile:30) 
2014-08-03 21:19:38 [INFO] [STDERR] at net.minecraft.world.biome.BiomeCache.func_76840_a(SourceFile:59) 
2014-08-03 21:19:38 [INFO] [STDERR] at net.minecraft.world.biome.BiomeCache.func_76837_b(SourceFile:68) 
2014-08-03 21:19:38 [INFO] [STDERR] at net.minecraft.world.biome.WorldChunkManager.func_76935_a(WorldChunkManager.java:67) 
2014-08-03 21:19:38 [INFO] [STDERR] at net.minecraft.world.World.getBiomeGenForCoordsBody(World.java:211) 
2014-08-03 21:19:38 [INFO] [STDERR] at net.minecraft.world.WorldProvider.getBiomeGenForCoords(WorldProvider.java:426) 
2014-08-03 21:19:38 [INFO] [STDERR] at net.minecraft.world.World.func_72807_a(World.java:196) 
2014-08-03 21:19:38 [INFO] [STDERR] at mbrx.ff.ecology.Volcanoes.doChunkTick(Volcanoes.java:50) 
2014-08-03 21:19:38 [INFO] [STDERR] at mbrx.ff.WorkerUpdateChunks.run(WorkerUpdateChunks.java:40) 
2014-08-03 21:19:38 [INFO] [STDERR] at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) 
2014-08-03 21:19:38 [INFO] [STDERR] at java.util.concurrent.FutureTask.run(Unknown Source) 
2014-08-03 21:19:38 [INFO] [STDERR] at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) 
2014-08-03 21:19:38 [INFO] [STDERR] at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) 
2014-08-03 21:19:38 [INFO] [STDERR] at java.lang.Thread.run(Unknown Source)

FF 0.5.9, FC 0.5.8