Hopper: java.lang.NullPointerException

This is an automatically generated report based from the following crash: http://hopper.minecraft.net/crashes/minecraft/MCX-190646/

Crash:

java.lang.NullPointerException
	at alq.f(SourceFile:197)
	at hn.m(SourceFile:669)
	at net.minecraft.server.MinecraftServer.a(SourceFile:874)
	at bkz.a(SourceFile:198)
	at mv.b(SourceFile:94)
	at net.minecraft.server.MinecraftServer.s(SourceFile:434)
	at bkz.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:340)
	at hi.run(SourceFile:583)

The following is the first clean report made public about this crash:

---- Minecraft Crash Report ----
// Shall we play a game?

Time: 9/19/13 4:30 PM
Description: Exception in server tick loop

java.lang.NullPointerException
	at alq.f(SourceFile:197)
	at hn.m(SourceFile:669)
	at net.minecraft.server.MinecraftServer.a(SourceFile:874)
	at bkz.a(SourceFile:198)
	at mv.b(SourceFile:94)
	at net.minecraft.server.MinecraftServer.s(SourceFile:434)
	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:
---------------------------------------------------------------------------------------

-- System Details --
Details:
	Minecraft Version: 1.6.4
	Operating System: Windows 7 (amd64) version 6.1
	Java Version: 1.6.0_23, Sun Microsystems Inc.
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Sun Microsystems Inc.
	Memory: 354302016 bytes (337 MB) / 632029184 bytes (602 MB) up to 954466304 bytes (910 MB)
	JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G
	AABB Pool Size: 1609 (90104 bytes; 0 MB) allocated, 58 (3248 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 188 (10528 bytes; 0 MB) allocated, 8 (448 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.

You can see this full report at http://hopper.minecraft.net/crashes/minecraft/MCX-190646/12432962/