Bukkit

What steps will reproduce the problem?
1. Killing a Mob at Full health I guess
2.
3.

What version of the product are you using?
Newest

Do you have an error log of what happened?

2012-09-21 15:42:52 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
org.bukkit.event.EventException
    at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:332)
    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 org.bukkit.craftbukkit.event.CraftEventFactory.callEntityDeathEvent(CraftEventFactory.java:299)
    at net.minecraft.server.EntityCreeper.dropDeathLoot(EntityCreeper.java:133)
    at net.minecraft.server.EntityLiving.die(EntityLiving.java:767)
    at net.minecraft.server.EntityCreeper.die(EntityCreeper.java:105)
    at net.minecraft.server.EntityLiving.damageEntity(EntityLiving.java:663)
    at net.minecraft.server.EntityMonster.damageEntity(EntityMonster.java:38)
    at net.minecraft.server.EntityArrow.h_(EntityArrow.java:222)
    at net.minecraft.server.World.entityJoinedWorld(World.java:1239)
    at net.minecraft.server.WorldServer.entityJoinedWorld(WorldServer.java:498)
    at net.minecraft.server.World.playerJoinedWorld(World.java:1221)
    at net.minecraft.server.World.tickEntities(World.java:1119)
    at net.minecraft.server.MinecraftServer.q(MinecraftServer.java:565)
    at net.minecraft.server.DedicatedServer.q(DedicatedServer.java:212)
    at net.minecraft.server.MinecraftServer.p(MinecraftServer.java:474)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:406)
    at net.minecraft.server.ThreadServerApplication.run(SourceFile:539)
Caused by: java.lang.IllegalArgumentException: Health must be between 0 and 20
    at org.bukkit.craftbukkit.entity.CraftLivingEntity.setHealth(CraftLivingEntity.java:58)
    at com.major.leechlife.EntityDeathListener.whenKilled(EntityDeathListener.java:58)
    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.java:330)
    ... 19 more

Please provide any additional information below.

Do check before how much health you can add  or if the player is already full?