Bukkit

What steps will reproduce the problem?
1. /booktotxt Feuerwerk
2.
3.

2012-12-30 17:02:32 [INFO] Giuseppe1990 issued server command: /booktotxt Feuerwerk
2012-12-30 17:02:32 [SEVERE] null
org.bukkit.command.CommandException: Unhandled exception executing command 'booktotxt' in plugin BookToTxt v1.0
    at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46)
    at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:186)
    at org.bukkit.craftbukkit.v1_4_6.CraftServer.dispatchCommand(CraftServer.java:510)
    at net.minecraft.server.v1_4_6.PlayerConnection.handleCommand(PlayerConnection.java:979)
    at net.minecraft.server.v1_4_6.PlayerConnection.chat(PlayerConnection.java:897)
    at net.minecraft.server.v1_4_6.PlayerConnection.a(PlayerConnection.java:852)
    at net.minecraft.server.v1_4_6.Packet3Chat.handle(Packet3Chat.java:44)
    at net.minecraft.server.v1_4_6.NetworkManager.b(NetworkManager.java:290)
    at net.minecraft.server.v1_4_6.PlayerConnection.d(PlayerConnection.java:112)
    at net.minecraft.server.v1_4_6.ServerConnection.b(SourceFile:39)
    at net.minecraft.server.v1_4_6.DedicatedServerConnection.b(SourceFile:30)
    at net.minecraft.server.v1_4_6.MinecraftServer.r(MinecraftServer.java:598)
    at net.minecraft.server.v1_4_6.DedicatedServer.r(DedicatedServer.java:224)
    at net.minecraft.server.v1_4_6.MinecraftServer.q(MinecraftServer.java:494)
    at net.minecraft.server.v1_4_6.MinecraftServer.run(MinecraftServer.java:427)
    at net.minecraft.server.v1_4_6.ThreadServerApplication.run(SourceFile:849)
Caused by: java.lang.NoClassDefFoundError: org/bukkit/craftbukkit/inventory/CraftItemStack
    at net.zhuoweizhang.booktotxt.BookToTxtPlugin.onCommand(BookToTxtPlugin.java:23)
    at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44)
    ... 15 more
Caused by: java.lang.ClassNotFoundException: org.bukkit.craftbukkit.inventory.CraftItemStack
    at org.bukkit.plugin.java.PluginClassLoader.findClass0(PluginClassLoader.java:70)
    at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:53)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
    ... 17 more
2012-12-30 17:02:36 [WARNING] Can't keep up! Did the system time change, or is the server overloaded?