Hopper: java.lang.NullPointerException 1.6.4

java.lang.NullPointerException
	at cn.b(SourceFile:116)
	at cn.a(SourceFile:41)

---- Minecraft Crash Report ----
// You should try our sister game, Minceraft!

Time: 19/09/13 12:57 PM
Description: Sending packet

java.lang.NullPointerException
	at cn.b(SourceFile:116)
	at cn.a(SourceFile:41)
	at ka.b(SourceFile:423)
	at hn.b(SourceFile:648)
	at hn.a(SourceFile:82)
	at blc.b(SourceFile:73)
	at net.minecraft.server.MinecraftServer.t(SourceFile:489)
	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 cn.b(SourceFile:116)
	at cn.a(SourceFile:41)

-- Packet being sent --
Details:
	Packet ID: 4
	Packet class: fx
Stacktrace:
	at ka.b(SourceFile:423)
	at hn.b(SourceFile:648)
	at hn.a(SourceFile:82)
	at blc.b(SourceFile:73)
	at net.minecraft.server.MinecraftServer.t(SourceFile:489)
	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 XP (x86) version 5.1
	Java Version: 1.7.0_25, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
	Memory: 150845768 bytes (143 MB) / 255938560 bytes (244 MB) up to 518979584 bytes (494 MB)
	JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M
	AABB Pool Size: 2121 (118776 bytes; 0 MB) allocated, 1943 (108808 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 1, tallocated: 63
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 2409 (134904 bytes; 0 MB) allocated, 2409 (134904 bytes; 0 MB) used
	Player Count: 0 / 8; []
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.