when loading a world i was previously using on mineacrft 1.6.4 the game craches, then when trying a second time again to lad the world it also crashes with these reports:

1st time crash
---- Minecraft Crash Report ----
// Why is it breaking

Time: 27/10/13 16:49
Description: Ticking memory connection

java.lang.IllegalArgumentException: n must be positive
at java.util.Random.nextInt(Unknown Source)
at agm.a(SourceFile:122)
at agm.a(SourceFile:27)
at agg.a(SourceFile:362)
at ahe.a(SourceFile:54)
at apl.a(SourceFile:432)
at mi.a(SourceFile:160)
at aoj.a(SourceFile:778)
at mi.c(SourceFile:95)
at mh.<init>(SourceFile:31)
at mg.a(SourceFile:212)
at mg.d(SourceFile:346)
at ld.d(SourceFile:215)
at mx.a(SourceFile:280)
at ir.a(SourceFile:137)
at is.a(SourceFile:63)
at ef.a(SourceFile:164)
at ms.c(SourceFile:134)
at net.minecraft.server.MinecraftServer.u(SourceFile:529)
at net.minecraft.server.MinecraftServer.t(SourceFile:441)
at bsk.t(SourceFile:117)
at net.minecraft.server.MinecraftServer.run(SourceFile:358)
at ky.run(SourceFile:617)

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

– Head –
Stacktrace:
at java.util.Random.nextInt(Unknown Source)
at agm.a(SourceFile:122)
at agm.a(SourceFile:27)
at agg.a(SourceFile:362)
at ahe.a(SourceFile:54)
at apl.a(SourceFile:432)
at mi.a(SourceFile:160)
at aoj.a(SourceFile:778)
at mi.c(SourceFile:95)
at mh.<init>(SourceFile:31)
at mg.a(SourceFile:212)
at mg.d(SourceFile:346)
at ld.d(SourceFile:215)
at mx.a(SourceFile:280)
at ir.a(SourceFile:137)
at is.a(SourceFile:63)
at ef.a(SourceFile:164)

– Ticking connection –
Details:
Connection: ef@7c163769
Stacktrace:
at ms.c(SourceFile:134)
at net.minecraft.server.MinecraftServer.u(SourceFile:529)
at net.minecraft.server.MinecraftServer.t(SourceFile:441)
at bsk.t(SourceFile:117)
at net.minecraft.server.MinecraftServer.run(SourceFile:358)
at ky.run(SourceFile:617)

– System Details –
Details:
Minecraft Version: 1.7.2
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.6.0_31, Sun Microsystems Inc.
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Sun Microsystems Inc.
Memory: 253267752 bytes (241 MB) / 493682688 bytes (470 MB) up to 954466304 bytes (910 MB)
JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G
AABB Pool Size: 6618 (370608 bytes; 0 MB) allocated, 6256 (350336 bytes; 0 MB) used
IntCache: cache: 5, tcache: 0, allocated: 13, tallocated: 95
Profiler Position: N/A (disabled)
Vec3 Pool Size: 1662 (93072 bytes; 0 MB) allocated, 1600 (89600 bytes; 0 MB) used
Player Count: 1 / 8; [mm['Mining_Man24'/206, l='anterna', x=335.94, y=71.60, z=-33.80]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.

2nd time:

---- Minecraft Crash Report ----
// I let you down. Sorry

Time: 27/10/13 16:49
Description: Ticking memory connection

java.lang.IllegalArgumentException: n must be positive
at java.util.Random.nextInt(Unknown Source)
at agm.a(SourceFile:122)
at agm.a(SourceFile:27)
at agg.a(SourceFile:362)
at ahe.a(SourceFile:54)
at apl.a(SourceFile:432)
at mi.a(SourceFile:160)
at aoj.a(SourceFile:781)
at mi.c(SourceFile:95)
at mh.<init>(SourceFile:31)
at mg.a(SourceFile:212)
at mg.a(SourceFile:249)
at ld.a(SourceFile:155)
at ld.c(SourceFile:206)
at ld.a(SourceFile:100)
at nd.c(SourceFile:87)
at nd.a(SourceFile:52)
at ef.a(SourceFile:166)
at ms.c(SourceFile:134)
at net.minecraft.server.MinecraftServer.u(SourceFile:529)
at net.minecraft.server.MinecraftServer.t(SourceFile:441)
at bsk.t(SourceFile:117)
at net.minecraft.server.MinecraftServer.run(SourceFile:358)
at ky.run(SourceFile:617)

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

– Head –
Stacktrace:
at java.util.Random.nextInt(Unknown Source)
at agm.a(SourceFile:122)
at agm.a(SourceFile:27)
at agg.a(SourceFile:362)
at ahe.a(SourceFile:54)
at apl.a(SourceFile:432)
at mi.a(SourceFile:160)
at aoj.a(SourceFile:781)
at mi.c(SourceFile:95)
at mh.<init>(SourceFile:31)
at mg.a(SourceFile:212)
at mg.a(SourceFile:249)
at ld.a(SourceFile:155)
at ld.c(SourceFile:206)
at ld.a(SourceFile:100)
at nd.c(SourceFile:87)
at nd.a(SourceFile:52)
at ef.a(SourceFile:166)

– Ticking connection –
Details:
Connection: ef@32ec1165
Stacktrace:
at ms.c(SourceFile:134)
at net.minecraft.server.MinecraftServer.u(SourceFile:529)
at net.minecraft.server.MinecraftServer.t(SourceFile:441)
at bsk.t(SourceFile:117)
at net.minecraft.server.MinecraftServer.run(SourceFile:358)
at ky.run(SourceFile:617)

– System Details –
Details:
Minecraft Version: 1.7.2
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.6.0_31, Sun Microsystems Inc.
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Sun Microsystems Inc.
Memory: 107349672 bytes (102 MB) / 289865728 bytes (276 MB) up to 954466304 bytes (910 MB)
JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G
AABB Pool Size: 2446 (136976 bytes; 0 MB) allocated, 2308 (129248 bytes; 0 MB) used
IntCache: cache: 5, tcache: 0, allocated: 13, tallocated: 95
Profiler Position: N/A (disabled)
Vec3 Pool Size: 322 (18032 bytes; 0 MB) allocated, 306 (17136 bytes; 0 MB) used
Player Count: 1 / 8; [mm['Mining_Man24'/206, l='anterna', x=335.94, y=71.60, z=-33.80]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.