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 act.a(SourceFile:1365)
	at kd.a(SourceFile:479)
	at act.g(SourceFile:1343)

---- Minecraft Crash Report ----
// Would you like a cupcake?

Time: 06-09-2013 14:36
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 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 act.a(SourceFile:1365)
	at kd.a(SourceFile:479)
	at act.g(SourceFile:1343)

-- Entity being ticked --
Details:
	Entity Type: Skeleton (ui)
	Entity ID: 32
	Entity Name: Skeleton
	Entity's Exact location: 946,50, 1,00, -473,50
	Entity's Block location: World: (946,1,-474), Chunk: (at 2,0,6 in 59,-30; contains blocks 944,0,-480 to 959,255,-465), Region: (1,-1; contains chunks 32,-32 to 63,-1, blocks 512,0,-512 to 1023,255,-1)
	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: New World
	All players: 1 total; [kg['xH4rdStyl3zx'/2, l='New World', x=947,34, y=1,00, z=-471,50]]
	Chunk stats: ServerChunkCache: 582 Drop: 0
	Level seed: 8078652195548766225
	Level generator: ID 01 - flat, ver 0. Features enabled: true
	Level generator options: 2;7;1;village
	Level spawn location: World: (930,4,-453), Chunk: (at 2,0,11 in 58,-29; contains blocks 928,0,-464 to 943,255,-449), Region: (1,-1; contains chunks 32,-32 to 63,-1, blocks 512,0,-512 to 1023,255,-1)
	Level time: 33552 game time, 1000 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 109187 (now: false), thunder time: 154354 (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: Linux (amd64) version 3.7.0-7-generic
	Java Version: 1.7.0_09, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 304989624 bytes (290 MB) / 527761408 bytes (503 MB) up to 954466304 bytes (910 MB)
	JVM Flags: 1 total; -Xmx1G
	AABB Pool Size: 225 (12600 bytes; 0 MB) allocated, 10 (560 bytes; 0 MB) used
	IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 15 (840 bytes; 0 MB) allocated, 2 (112 bytes; 0 MB) used
	Player Count: 1 / 8; [kg['xH4rdStyl3zx'/2, l='New World', x=947,34, y=1,00, z=-471,50]]
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.