Hopper: java.lang.NullPointerException

java.lang.NullPointerException
	at ou.b(SourceFile:536)
	at ou.aL(SourceFile:399)
	at ou.z(SourceFile:222)
	at ov.z(SourceFile:128)
	at oc.o_(SourceFile:279)
	at ou.o_(SourceFile:1167)
	at ov.o_(SourceFile:169)
	at ud.o_(SourceFile:29)
	at un.o_(SourceFile:211)
	at act.a(SourceFile:1365)
	at kd.a(SourceFile:479)
	at act.g(SourceFile:1343)

---- Minecraft Crash Report ----
// I just don't know what went wrong :(

Time: 07.09.13 17:30
Description: Ticking entity

java.lang.NullPointerException
	at ou.b(SourceFile:536)
	at ou.aL(SourceFile:399)
	at ou.z(SourceFile:222)
	at ov.z(SourceFile:128)
	at oc.o_(SourceFile:279)
	at ou.o_(SourceFile:1167)
	at ov.o_(SourceFile:169)
	at ud.o_(SourceFile:29)
	at un.o_(SourceFile:211)
	at act.a(SourceFile:1365)
	at kd.a(SourceFile:479)
	at act.g(SourceFile:1343)
	at act.h(SourceFile:1250)
	at kd.h(SourceFile:364)
	at net.minecraft.server.MinecraftServer.t(SourceFile:463)
	at net.minecraft.server.MinecraftServer.s(SourceFile:398)
	at bnn.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:332)
	at hr.run(SourceFile:575)


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

-- Head --
Stacktrace:
	at ou.b(SourceFile:536)
	at ou.aL(SourceFile:399)
	at ou.z(SourceFile:222)
	at ov.z(SourceFile:128)
	at oc.o_(SourceFile:279)
	at ou.o_(SourceFile:1167)
	at ov.o_(SourceFile:169)
	at ud.o_(SourceFile:29)
	at un.o_(SourceFile:211)
	at act.a(SourceFile:1365)
	at kd.a(SourceFile:479)
	at act.g(SourceFile:1343)

-- Entity being ticked --
Details:
	Entity Type: Zombie (un)
	Entity ID: 2368
	Entity Name: Zombie
	Entity's Exact location: -1153,50, 58,00, -816,50
	Entity's Block location: World: (-1154,58,-817), Chunk: (at 14,3,15 in -73,-52; contains blocks -1168,0,-832 to -1153,255,-817), Region: (-3,-2; contains chunks -96,-64 to -65,-33, blocks -1536,0,-1024 to -1025,255,-513)
	Entity's Momentum: 0,00, 0,00, 0,00
Stacktrace:
	at act.h(SourceFile:1250)
	at kd.h(SourceFile:364)

-- Affected level --
Details:
	Level name: TEST SOME SPAWNERS
	All players: 1 total; [kg['_RuRo_'/53, l='TEST SOME SPAWNERS', x=-1155,10, y=53,00, z=-814,99]]
	Chunk stats: ServerChunkCache: 625 Drop: 0
	Level seed: -7677568096856158022
	Level generator: ID 01 - flat, ver 0. Features enabled: true
	Level generator options: 2;7,52x24;1;village
	Level spawn location: World: (-1150,4,-814), Chunk: (at 2,0,2 in -72,-51; contains blocks -1152,0,-816 to -1137,255,-801), Region: (-3,-2; contains chunks -96,-64 to -65,-33, blocks -1536,0,-1024 to -1025,255,-513)
	Level time: 3430 game time, 3430 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 15516 (now: false), thunder time: 147353 (now: false)
	Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: true
Stacktrace:
	at net.minecraft.server.MinecraftServer.t(SourceFile:463)
	at net.minecraft.server.MinecraftServer.s(SourceFile:398)
	at bnn.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:332)
	at hr.run(SourceFile:575)

-- System Details --
Details:
	Minecraft Version: 13w36b
	Operating System: Mac OS X (x86_64) version 10.7.5
	Java Version: 1.6.0_41, Apple Inc.
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
	Memory: 113608432 bytes (108 MB) / 280891392 bytes (267 MB) up to 1060372480 bytes (1011 MB)
	JVM Flags: 1 total; -Xmx1G
	AABB Pool Size: 875 (49000 bytes; 0 MB) allocated, 517 (28952 bytes; 0 MB) used
	IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 132 (7392 bytes; 0 MB) allocated, 121 (6776 bytes; 0 MB) used
	Player Count: 1 / 8; [kg['_RuRo_'/53, l='TEST SOME SPAWNERS', x=-1155,10, y=53,00, z=-814,99]]
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.