Biome crash superflat

I was just flying around my adventure map world when this happened. I am serious, I have absolutely no idea why this happened.

Description: Getting biome

java.lang.ArrayIndexOutOfBoundsException: 15
	at java.util.ArrayList.add(Unknown Source)
	at ark.a(SourceFile:55)
	at ark.a(SourceFile:62)
	at ars.a(SourceFile:64)
	at bfa.a(SourceFile:1020)
	at aqn.b(SourceFile:161)
	at arc.b(SourceFile:158)
	at arm.a(SourceFile:36)
	at arm.a(SourceFile:47)
	at awg.a(SourceFile:49)