playerskull.error

[SEVERE] Could not load 'plugins\PlayerSkulls.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.NoClassDefFoundError: net/minecraft/server/NBTBase
    at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:184)
    at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:305)
    at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:230)
    at org.bukkit.craftbukkit.v1_4_6.CraftServer.loadPlugins(CraftServer.java:235)
    at org.bukkit.craftbukkit.v1_4_6.CraftServer.<init>(CraftServer.java:213)
    at net.minecraft.server.v1_4_6.PlayerList.<init>(PlayerList.java:52)
    at net.minecraft.server.v1_4_6.DedicatedPlayerList.<init>(SourceFile:11)
    at net.minecraft.server.v1_4_6.DedicatedServer.init(DedicatedServer.java:104)
    at net.minecraft.server.v1_4_6.MinecraftServer.run(MinecraftServer.java:399)
    at net.minecraft.server.v1_4_6.ThreadServerApplication.run(SourceFile:849)
Caused by: java.lang.NoClassDefFoundError: net/minecraft/server/NBTBase
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Unknown Source)
    at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:173)
    ... 9 more
Caused by: java.lang.ClassNotFoundException: net.minecraft.server.NBTBase
    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(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    ... 12 more