[SEVERE] Encountered an unexpected exception ReportedException
net.minecraft.util.ReportedException: Exception ticking world
	at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:646)
	at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:274)
	at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:570)
	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:468)
	at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:573)
Caused by: java.lang.IllegalArgumentException
	at net.minecraft.util.WeightedRandom.func_76273_a(SourceFile:17)
	at net.minecraft.util.WeightedRandom.func_76271_a(SourceFile:31)
	at net.minecraft.world.WorldServer.func_73057_a(WorldServer.java:217)
	at net.minecraft.world.SpawnerAnimals.func_77192_a(SpawnerAnimals.java:159)
	at net.minecraft.world.WorldServer.func_72835_b(WorldServer.java:177)
	at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:640)
	... 4 more
 [SEVERE] This crash report has been saved to: C:\Users\Courtney\Desktop\MC 1.5 Version\.\crash-reports\crash-2013-04-07_00.40.10-server.txt