Hopper: java.lang.ClassCastException: cannot be cast to bkf

java.lang.ClassCastException:  cannot be cast to bkf
	at bkg.a(SourceFile:139)
	at bll.a(SourceFile:991)

---- Minecraft Crash Report ----
// Why did you do that?

Time: 29.10.13 16:14
Description: Unexpected error

java.lang.ClassCastException:   cannot be cast to bkf
	at bkg.a(SourceFile:139)
	at bll.a(SourceFile:991)
	at bll.b(SourceFile:813)
	at azd.ad(SourceFile:780)
	at azd.e(SourceFile:704)
	at net.minecraft.client.main.Main.main(SourceFile:103)


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

-- Head --
Stacktrace:
	at bkg.a(SourceFile:139)
	at bll.a(SourceFile:991)

-- Affected level --
Details:
	Level name: MpServer
	All players: 1 total; [bje['SilverSzymon'/203, l='MpServer', x=-41,51, y=47,62, z=537,06]]
	Chunk stats: MultiplayerChunkCache: 240, 240
	Level seed: 0
	Level generator: ID 00 - default, ver 1. Features enabled: false
	Level generator options: 
	Level spawn location: World: (-20,64,252), Chunk: (at 12,4,12 in -2,15; contains blocks -32,0,240 to -17,255,255), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
	Level time: 30623 game time, 4031 day time
	Level dimension: 0
	Level storage version: 0x00000 - Unknown?
	Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
	Forced entities: 36 total; [vw['item.tile.stonebrick'/10395, l='MpServer', x=-41,13, y=44,13, z=538,78], us['Kura'/273, l='MpServer', x=-11,50, y=70,00, z=507,50], us['Kura'/272, l='MpServer', x=-11,53, y=63,00, z=499,47], uq['Nietoperz'/6259, l='MpServer', x=-115,41, y=23,86, z=539,56], vw['item.tile.stonebrick'/10573, l='MpServer', x=-40,22, y=45,13, z=538,44], ve['Kałamarnica'/8200, l='MpServer', x=-45,50, y=60,31, z=490,09], uq['Nietoperz'/3703, l='MpServer', x=-21,00, y=30,95, z=518,56], ut['Krowa'/258, l='MpServer', x=-54,16, y=71,00, z=571,19], vw['item.tile.stonebrick'/10789, l='MpServer', x=-41,88, y=44,13, z=538,13], ut['Krowa'/257, l='MpServer', x=-53,22, y=71,00, z=569,22], uq['Nietoperz'/1900, l='MpServer', x=-33,28, y=32,10, z=515,00], ve['Kałamarnica'/263, l='MpServer', x=-32,45, y=54,52, z=512,53], us['Kura'/260, l='MpServer', x=-39,44, y=71,00, z=520,41], uq['Nietoperz'/8993, l='MpServer', x=-71,09, y=17,93, z=613,01], uq['Nietoperz'/8645, l='MpServer', x=-89,54, y=19,72, z=527,53], ve['Kałamarnica'/266, l='MpServer', x=-30,47, y=54,00, z=511,53], uq['Nietoperz'/8644, l='MpServer', x=-96,49, y=12,72, z=555,50], ut['Krowa'/267, l='MpServer', x=-24,16, y=71,00, z=574,03], ve['Kałamarnica'/264, l='MpServer', x=-24,88, y=57,00, z=508,50], ve['Kałamarnica'/265, l='MpServer', x=-31,71, y=54,37, z=510,70], ut['Krowa'/270, l='MpServer', x=-27,25, y=71,00, z=570,47], uq['Nietoperz'/8640, l='MpServer', x=-28,75, y=31,10, z=517,50], uq['Nietoperz'/8643, l='MpServer', x=-103,67, y=20,00, z=531,53], ut['Krowa'/268, l='MpServer', x=-29,81, y=72,00, z=568,19], ut['Krowa'/269, l='MpServer', x=-25,50, y=71,00, z=570,47], us['Kura'/239, l='MpServer', x=-96,66, y=72,00, z=612,56], us['Kura'/238, l='MpServer', x=-103,50, y=71,00, z=615,50], us['Kura'/236, l='MpServer', x=-104,41, y=71,00, z=607,63], uq['Nietoperz'/8977, l='MpServer', x=-52,46, y=43,00, z=486,93], bje['SilverSzymon'/203, l='MpServer', x=-41,51, y=47,62, z=537,06], uq['Nietoperz'/8976, l='MpServer', x=-35,76, y=40,06, z=491,30], vw['item.item.egg'/10592, l='MpServer', x=-49,12, y=69,13, z=496,72], vw['item.item.egg'/9865, l='MpServer', x=-35,00, y=69,13, z=510,53], us['Kura'/255, l='MpServer', x=-49,56, y=70,00, z=495,63], vb['Owca'/290, l='MpServer', x=31,97, y=71,00, z=464,06], us['Kura'/240, l='MpServer', x=-101,50, y=72,00, z=612,50]]
	Retry entities: 0 total; []
	Server brand: vanilla
	Server type: Integrated singleplayer server
Stacktrace:
	at biz.a(SourceFile:284)
	at azd.b(SourceFile:1951)
	at azd.e(SourceFile:718)
	at net.minecraft.client.main.Main.main(SourceFile:103)

-- System Details --
Details:
	Minecraft Version: 1.7.2
	Operating System: Windows 7 (x86) version 6.1
	Java Version: 1.7.0_45, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
	Memory: 50706328 bytes (48 MB) / 193392640 bytes (184 MB) up to 518979584 bytes (494 MB)
	JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M
	AABB Pool Size: 1046 (58576 bytes; 0 MB) allocated, 621 (34776 bytes; 0 MB) used
	IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 95
	Launched Version: 1.7.2
	LWJGL: 2.9.0
	OpenGL: GeForce GTX 550 Ti/PCIe/SSE2 GL version 4.4.0, NVIDIA Corporation
	Is Modded: Probably not. Jar signature remains and client brand is untouched.
	Type: Client (map_client.txt)
	Resource Packs: []
	Current Language: Polski (Polska)
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 345 (19320 bytes; 0 MB) allocated, 303 (16968 bytes; 0 MB) used
	Anisotropic Filtering: Off (1)