Hopper: java.lang.NullPointerException

java.lang.NullPointerException
	at asp.a(SourceFile:168)
	at abw.h(SourceFile:1272)
	at js.h(SourceFile:364)

---- Minecraft Crash Report ----
// I feel sad now :(

Time: 9/20/13 1:55 PM
Description: Exception ticking world entities

java.lang.NullPointerException
	at asp.a(SourceFile:168)
	at abw.h(SourceFile:1272)
	at js.h(SourceFile:364)
	at net.minecraft.server.MinecraftServer.t(SourceFile:471)
	at net.minecraft.server.MinecraftServer.s(SourceFile:406)
	at bkz.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:340)
	at hi.run(SourceFile:583)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Stacktrace:
	at asp.a(SourceFile:168)
	at abw.h(SourceFile:1272)
	at js.h(SourceFile:364)

-- Affected level --
Details:
	Level name: tornado?
	All players: 1 total; [jv['horses_rock519'/200635, l='tornado?', x=163.99, y=71.17, z=654.33]]
	Chunk stats: ServerChunkCache: 1081 Drop: 0
	Level seed: 2283534770024377437
	Level generator: ID 00 - default, ver 1. Features enabled: true
	Level generator options: 
	Level spawn location: World: (108,64,256), Chunk: (at 12,4,0 in 6,16; contains blocks 96,0,256 to 111,255,271), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
	Level time: 16381 game time, 16381 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 86502 (now: false), thunder time: 7489 (now: true)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
	at net.minecraft.server.MinecraftServer.t(SourceFile:471)
	at net.minecraft.server.MinecraftServer.s(SourceFile:406)
	at bkz.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:340)
	at hi.run(SourceFile:583)

-- System Details --
Details:
	Minecraft Version: 1.6.4
	Operating System: Windows 7 (x86) version 6.1
	Java Version: 1.7.0_09, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
	Memory: 128236648 bytes (122 MB) / 374632448 bytes (357 MB) up to 518979584 bytes (494 MB)
	JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M
	AABB Pool Size: 6314 (353584 bytes; 0 MB) allocated, 5732 (320992 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 3, tallocated: 63
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 2060 (115360 bytes; 0 MB) allocated, 1858 (104048 bytes; 0 MB) used
	Player Count: 1 / 8; [jv['horses_rock519'/200635, l='tornado?', x=163.99, y=71.17, z=654.33]]
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.