Hopper: java.lang.NullPointerException

java.lang.NullPointerException
	at hn.a(SourceFile:154)
	at hn.a(SourceFile:54)
	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)

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

Time: 9/20/13 10:49 AM
Description: Exception in server tick loop

java.lang.NullPointerException
	at hn.a(SourceFile:154)
	at hn.a(SourceFile:54)
	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:
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-- System Details --
Details:
	Minecraft Version: 1.6.4
	Operating System: Linux (amd64) version 3.8.0-30-generic
	Java Version: 1.7.0_25, Oracle Corporation
	Java VM Version: OpenJDK 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 98790776 bytes (94 MB) / 254083072 bytes (242 MB) up to 954466304 bytes (910 MB)
	JVM Flags: 1 total; -Xmx1G
	AABB Pool Size: 208 (11648 bytes; 0 MB) allocated, 198 (11088 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: 25 (1400 bytes; 0 MB) allocated, 25 (1400 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.