Citizens2 Conflict

10:23:10 [SEVERE] Could not pass event EntityDamageEvent to Skillz vjenkins-Skillz-219
org.bukkit.event.EventException
        at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:427)
        at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62)
        at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:477)
        at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:462)
        at net.minecraft.server.v1_4_R1.Entity.y(Entity.java:324)
        at net.minecraft.server.v1_4_R1.EntityLiving.y(EntityLiving.java:310)
        at net.minecraft.server.v1_4_R1.Entity.j_(Entity.java:241)
        at net.minecraft.server.v1_4_R1.EntityLiving.j_(EntityLiving.java:508)
        at net.minecraft.server.v1_4_R1.EntityMonster.j_(EntityMonster.java:24)
        at net.minecraft.server.v1_4_R1.EntityZombie.j_(EntityZombie.java:121)
        at net.minecraft.server.v1_4_R1.World.entityJoinedWorld(World.java:1332)

        at net.minecraft.server.v1_4_R1.WorldServer.entityJoinedWorld(WorldServer.java:548)
        at net.minecraft.server.v1_4_R1.World.playerJoinedWorld(World.java:1313)

        at net.minecraft.server.v1_4_R1.World.tickEntities(World.java:1191)
        at net.minecraft.server.v1_4_R1.WorldServer.tickEntities(WorldServer.java:445)
        at net.minecraft.server.v1_4_R1.MinecraftServer.r(MinecraftServer.java:580)
        at net.minecraft.server.v1_4_R1.DedicatedServer.r(DedicatedServer.java:224)
        at net.minecraft.server.v1_4_R1.MinecraftServer.q(MinecraftServer.java:494)
        at net.minecraft.server.v1_4_R1.MinecraftServer.run(MinecraftServer.java:427)
        at net.minecraft.server.v1_4_R1.ThreadServerApplication.run(SourceFile:849)
Caused by: java.lang.NoClassDefFoundError: net/citizensnpcs/api/CitizensManager
        at nl.lolmen.Skills.SkillEntityListener.onEntityDamage(SkillEntityListener.java:35)
        at sun.reflect.GeneratedMethodAccessor84.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.java:425)
        ... 19 more