stackableitems error

15:43:53 [SEVERE] Could not pass event PlayerPickupItemEvent to StackableItems
org.bukkit.event.EventException
        at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.ja
va:324)
        at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.jav
a:62)
        at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.j
ava:477)
        at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.j
ava:462)
        at net.minecraft.server.EntityItem.b_(EntityItem.java:193)
        at net.minecraft.server.EntityHuman.o(EntityHuman.java:349)
        at net.minecraft.server.EntityHuman.d(EntityHuman.java:341)
        at net.minecraft.server.EntityLiving.h_(EntityLiving.java:447)
        at net.minecraft.server.EntityHuman.h_(EntityHuman.java:158)
        at net.minecraft.server.EntityPlayer.g(EntityPlayer.java:203)
        at net.minecraft.server.NetServerHandler.d(NetServerHandler.java:128)
        at net.minecraft.server.ServerConnection.b(SourceFile:35)
        at net.minecraft.server.DedicatedServerConnection.b(SourceFile:30)
        at net.minecraft.server.MinecraftServer.q(MinecraftServer.java:583)
        at net.minecraft.server.DedicatedServer.q(DedicatedServer.java:212)
        at net.minecraft.server.MinecraftServer.p(MinecraftServer.java:476)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:408)
        at net.minecraft.server.ThreadServerApplication.run(SourceFile:539)
Caused by: java.lang.NoClassDefFoundError: org/bukkit/Sound
        at haveric.stackableItems.SIPlayerListener.playerPicksUpItem(SIPlayerLis
tener.java:1058)
        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 org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.ja
va:322)
        ... 17 more