Bukkit

In my server i am the op so i have all the permissions to do every command but when i type /md it says
"An internl error occured while attempting to perform this command. and when i check the console. it spams this

12:14:32 [WARNING] Task of 'MobDisguise' generated an exception
java.lang.NoSuchFieldError: f
        at me.desmin88.mobdisguise.utils.PacketUtils.packetMaker(PacketUtils.jav
a:146)
        at me.desmin88.mobdisguise.utils.PacketUtils.disguiseToAll(PacketUtils.j
ava:61)
        at me.desmin88.mobdisguise.utils.DisguiseTask.run(DisguiseTask.java:25)
12:14:32 [WARNING] Task of 'MobDisguise' generated an exception
java.lang.NoSuchFieldError: f
        at me.desmin88.mobdisguise.utils.PacketUtils.packetMaker(PacketUtils.jav
a:146)
        at me.desmin88.mobdisguise.utils.PacketUtils.disguiseToAll(PacketUtils.j
ava:61)
        at me.desmin88.mobdisguise.utils.DisguiseTask.run(DisguiseTask.java:25)
12:14:34 [WARNING] Task of 'MobDisguise' generated an exception
java.lang.NoSuchFieldError: f
        at me.desmin88.mobdisguise.utils.PacketUtils.packetMaker(PacketUtils.jav
a:146)
        at me.desmin88.mobdisguise.utils.PacketUtils.disguiseToAll(PacketUtils.j
ava:61)
        at me.desmin88.mobdisguise.utils.DisguiseTask.run(DisguiseTask.java:25)
12:14:35 [WARNING] Azoka1 moved wrongly!
12:14:36 [WARNING] Task of 'MobDisguise' generated an exception
java.lang.NoSuchFieldError: f
        at me.desmin88.mobdisguise.utils.PacketUtils.packetMaker(PacketUtils.jav
a:146)
        at me.desmin88.mobdisguise.utils.PacketUtils.disguiseToAll(PacketUtils.j
ava:61)
        at me.desmin88.mobdisguise.utils.DisguiseTask.run(DisguiseTask.java:25)
12:14:37 [WARNING] Task of 'MobDisguise' generated an exception
java.lang.NoSuchFieldError: f
        at me.desmin88.mobdisguise.utils.PacketUtils.packetMaker(PacketUtils.jav
a:146)
        at me.desmin88.mobdisguise.utils.PacketUtils.disguiseToAll(PacketUtils.j
ava:61)
        at me.desmin88.mobdisguise.utils.DisguiseTask.run(DisguiseTask.java:25)
It keeps spamming this and it fills up my console. Please tell me how to fix this.
But the weird thing is that when i type /md types it tells me the types of mobs i can disguise into. And the jar also generated a config in my plugins folder.