says the crash was due to a modded client but i havent modded anything. felt a responsibility to report the crash.

---- Minecraft Crash Report ----
// You're mean.

Time: 23/09/13 23:00
Description: Comparing biomes

java.lang.IllegalArgumentException: Biome ID is out of bounds: 2563
at aea.d(SourceFile:480)
at ana.a(SourceFile:159)
at ane.a(SourceFile:67)
at and.a(SourceFile:13)
at anp.a(SourceFile:20)
at amo.a(SourceFile:16)
at anp.a(SourceFile:20)
at anj.a(SourceFile:13)
at anp.a(SourceFile:20)
at anp.a(SourceFile:20)
at ank.a(SourceFile:15)
at ani.a(SourceFile:24)
at ano.a(SourceFile:23)
at aeh.a(SourceFile:165)
at aef.<init>(SourceFile:27)
at aee.a(SourceFile:52)
at aee.b(SourceFile:61)
at aeh.a(SourceFile:61)
at afk.a(SourceFile:930)
at ade.a(SourceFile:103)
at ade.a(SourceFile:1029)
at bii.i(SourceFile:1316)
at bii.a(SourceFile:912)
at bii.b(SourceFile:807)
at awp.aa(SourceFile:754)
at awp.e(SourceFile:677)
at net.minecraft.client.main.Main.main(SourceFile:104)

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

– Head –
Stacktrace:
at aea.d(SourceFile:480)

– Biomes being compared –
Details:
Biome A ID: 2563
Biome B ID: 38
Biome A: ~_ERROR~ IllegalArgumentException: Biome ID is out of bounds: 2563
Biome B: aer@5a85ce81
Stacktrace:
at ana.a(SourceFile:159)
at ane.a(SourceFile:67)
at and.a(SourceFile:13)
at anp.a(SourceFile:20)
at amo.a(SourceFile:16)
at anp.a(SourceFile:20)
at anj.a(SourceFile:13)
at anp.a(SourceFile:20)
at anp.a(SourceFile:20)
at ank.a(SourceFile:15)
at ani.a(SourceFile:24)
at ano.a(SourceFile:23)
at aeh.a(SourceFile:165)
at aef.<init>(SourceFile:27)
at aee.a(SourceFile:52)
at aee.b(SourceFile:61)
at aeh.a(SourceFile:61)
at afk.a(SourceFile:930)

– Coordinates of biome request –
Details:
Location: World: (-1587,0,-910), Chunk: (at 13,0,2 in -100,-57; contains blocks -1600,0,-912 to -1585,255,-897), Region: (-4,-2; contains chunks -128,-64 to -97,-33, blocks -2048,0,-1024 to -1537,255,-513)
Stacktrace:
at ade.a(SourceFile:103)
at ade.a(SourceFile:1029)
at bii.i(SourceFile:1316)
at bii.a(SourceFile:912)

– Affected level –
Details:
Level name: MpServer
All players: 1 total; [bgh['Lestti'/2358173, l='MpServer', x=-1586.50, y=66.72, z=-909.50]]
Chunk stats: MultiplayerChunkCache: 0, 0
Level seed: 0
Level generator: ID 00 - default, ver 1. Features enabled: false
Level generator options:
Level spawn location: World: (-72,64,244), Chunk: (at 8,4,4 in -5,15; contains blocks -80,0,240 to -65,255,255), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
Level time: 0 game time, 0 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: 1 total; [bgh['Lestti'/2358173, l='MpServer', x=-1586.50, y=66.72, z=-909.50]]
Retry entities: 0 total; []
Server brand: vanilla
Server type: Integrated singleplayer server
Stacktrace:
at bgc.a(SourceFile:297)
at awp.b(SourceFile:1874)
at awp.e(SourceFile:686)
at net.minecraft.client.main.Main.main(SourceFile:104)

– System Details –
Details:
Minecraft Version: 13w38c
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.7.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 343801880 bytes (327 MB) / 576585728 bytes (549 MB) up to 954466304 bytes (910 MB)
JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G
AABB Pool Size: 10738 (601328 bytes; 0 MB) allocated, 10738 (601328 bytes; 0 MB) used
IntCache: cache: 0, tcache: 34, allocated: 12, tallocated: 68
Launched Version: 13w38c
LWJGL: 2.9.0
OpenGL: GeForce GTX 650/PCIe/SSE2 GL version 4.3.0, NVIDIA Corporation
Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Resource Packs: [1.6 Docm77 Resource Pack v2.zip]
Current Language: English (US)
Profiler Position: N/A (disabled)
Vec3 Pool Size: 11 (616 bytes; 0 MB) allocated, 11 (616 bytes; 0 MB) used