Minecraft crashed

---- Minecraft Crash Report ----
// This doesn't make any sense!

Time: 4/23/13 1:23 PM
Description: Exception in server tick loop

java.lang.VerifyError: (class: jc, method: c signature: (III)V) Bad type in putfield/putstatic
	at gu.a(SourceFile:271)
	at bjk.b(SourceFile:67)
	at net.minecraft.server.MinecraftServer.r(SourceFile:480)
	at net.minecraft.server.MinecraftServer.q(SourceFile:397)
	at bjh.q(SourceFile:122)
	at net.minecraft.server.MinecraftServer.run(SourceFile:331)
	at gp.run(SourceFile:573)


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

-- System Details --
Details:
	Minecraft Version: 1.5.1
	Operating System: Windows 7 (x86) version 6.1
	Java Version: 1.7.0_17, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
	Memory: 357691840 bytes (341 MB) / 519110656 bytes (495 MB) up to 1037959168 bytes (989 MB)
	JVM Flags: 2 total; -Xms512m -Xmx1024m
	AABB Pool Size: 3268 (183008 bytes; 0 MB) allocated, 3055 (171080 bytes; 0 MB) used
	Suspicious classes: [com.thevoxelbox.voxelmap.VoxelMap, VoxelRadar, VoxelColorManager, ...], [com.thevoxelbox.voxelmap.util.GuiScreenMinimap, GuiMinimap, GuiScreenAddWaypoint, ...]
	IntCache: cache: 0, tcache: 0, allocated: 1, tallocated: 63
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 706 (39536 bytes; 0 MB) allocated, 701 (39256 bytes; 0 MB) used
	Player Count: 0 / 8; []
	Type: Integrated Server (map_client.txt)
	Is Modded: Very likely; Jar signature invalidated