Hola, quisiera saber por que me da el siguiente error:

En Minecraft 1.4.6 todo me iba bien, descargué el Forge y me cargaba tranquilamente, pero le puse un MOD y me carga pero me sale un Crash. Le contenido:

—- Minecraft Crash Report —-
// Surprise! Haha. Well, this is awkward.

Time: 27/09/13 22:49
Description: Initializing game

java.lang.NoClassDefFoundError: Ldrzhark/customspawner/CustomSpawner;
at java.lang.Class.getDeclaredFields0(Native Method)
at java.lang.Class.privateGetDeclaredFields(Unknown Source)
at java.lang.Class.getDeclaredField(Unknown Source)
at cpw.mods.fml.common.ProxyInjector.inject(ProxyInjector.java:43)
at cpw.mods.fml.common.FMLModContainer.constructMod(FMLModContainer.java:519)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:313)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
at com.google.common.eventbus.EventBus.post(EventBus.java:267)
at cpw.mods.fml.common.LoadController.sendEventToModContainer(LoadController.java:194)
at cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java:174)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:313)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
at com.google.common.eventbus.EventBus.post(EventBus.java:267)
at cpw.mods.fml.common.LoadController.distributeStateMessage(LoadController.java:105)
at cpw.mods.fml.common.Loader.loadMods(Loader.java:509)
at cpw.mods.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:181)
at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:471)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:806)
at net.minecraft.client.main.Main.main(SourceFile:101)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:131)
at net.minecraft.launchwrapper.Launch.main(Launch.java:27)
Caused by: java.lang.ClassNotFoundException: drzhark.customspawner.CustomSpawner
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:179)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
… 37 more
Caused by: java.lang.NullPointerException
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:171)
… 39 more

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

— Head —
Stacktrace:
at java.lang.Class.getDeclaredFields0(Native Method)
at java.lang.Class.privateGetDeclaredFields(Unknown Source)
at java.lang.Class.getDeclaredField(Unknown Source)
at cpw.mods.fml.common.ProxyInjector.inject(ProxyInjector.java:43)
at cpw.mods.fml.common.FMLModContainer.constructMod(FMLModContainer.java:519)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:313)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
at com.google.common.eventbus.EventBus.post(EventBus.java:267)
at cpw.mods.fml.common.LoadController.sendEventToModContainer(LoadController.java:194)
at cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java:174)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:313)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
at com.google.common.eventbus.EventBus.post(EventBus.java:267)
at cpw.mods.fml.common.LoadController.distributeStateMessage(LoadController.java:105)
at cpw.mods.fml.common.Loader.loadMods(Loader.java:509)
at cpw.mods.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:181)
at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:471)

— Initialization —
Details:
Stacktrace:
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:806)
at net.minecraft.client.main.Main.main(SourceFile:101)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:131)
at net.minecraft.launchwrapper.Launch.main(Launch.java:27)

— System Details —
Details:
Minecraft Version: 1.6.4
Operating System: Windows 7 (x86) version 6.1
Java Version: 1.7.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
Memory: 34406680 bytes (32 MB) / 127836160 bytes (121 MB) up to 518979584 bytes (494 MB)
JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M
AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
Suspicious classes: FML and Forge are installed
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP v8.11 FML v6.4.6.884 Minecraft Forge 9.11.0.884 4 mods loaded, 4 mods active
mcp{8.09} [Minecraft Coder Pack] (minecraft.jar) Unloaded->Constructed
FML{6.4.6.884} [Forge Mod Loader] (minecraftforge-9.11.0.884.jar) Unloaded->Constructed
Forge{9.11.0.884} [Minecraft Forge] (minecraftforge-9.11.0.884.jar) Unloaded->Constructed
MoCreatures{6.0.1} [DrZhark’s Mo’Creatures Mod] (DrZharks MoCreatures Mod v6.0.1.zip) Unloaded
Launched Version: 1.6.4-Forge9.11.0.884
LWJGL: 2.9.0
OpenGL: Intel(R) HD Graphics 3000 GL version 3.1.0 – Build 9.17.10.2932, Intel
Is Modded: Definitely; Client brand changed to ‘fml,forge’
Type: Client (map_client.txt)
Resource Pack: Default
Current Language: Español (España)
Profiler Position: N/A (disabled)
Vec3 Pool Size: ~~ERROR~~ NullPointerException: null

No sé que significa, ayúdenme a resolverlo. ¿Vuelvo a 1.6.2?