InfernalMobs Error

[14:42:59] [Server thread/ERROR]: Could not pass event CreatureSpawnEvent to InfernalMobs v5.4
org.bukkit.event.EventException
	at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:310) ~[classic.jar:git-Spigot-21fe707-e1ebe52]
	at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[classic.jar:git-Spigot-21fe707-e1ebe52]
	at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [classic.jar:git-Spigot-21fe707-e1ebe52]
	at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [classic.jar:git-Spigot-21fe707-e1ebe52]
	at org.bukkit.craftbukkit.v1_8_R3.event.CraftEventFactory.callCreatureSpawnEvent(CraftEventFactory.java:277) [classic.jar:git-Spigot-21fe707-e1ebe52]
	at net.minecraft.server.v1_8_R3.World.addEntity(World.java:1015) [classic.jar:git-Spigot-21fe707-e1ebe52]
	at org.bukkit.craftbukkit.v1_8_R3.CraftWorld.addEntity(CraftWorld.java:1129) [classic.jar:git-Spigot-21fe707-e1ebe52]
	at org.bukkit.craftbukkit.v1_8_R3.CraftWorld.spawn(CraftWorld.java:1136) [classic.jar:git-Spigot-21fe707-e1ebe52]
	at org.bukkit.craftbukkit.v1_8_R3.CraftWorld.spawn(CraftWorld.java:859) [classic.jar:git-Spigot-21fe707-e1ebe52]
	at org.bukkit.craftbukkit.v1_8_R3.CraftWorld.spawnEntity(CraftWorld.java:378) [classic.jar:git-Spigot-21fe707-e1ebe52]
	at org.matecraft.kodaichi.coliseumcraft.ColiseumHelper.spawnMob(ColiseumHelper.java:416) [ColiseumCraft-%5B1.4%5D.jar:?]
	at org.matecraft.kodaichi.coliseumcraft.ActiveColiseum.spawnWave(ActiveColiseum.java:1160) [ColiseumCraft-%5B1.4%5D.jar:?]
	at org.matecraft.kodaichi.coliseumcraft.ActiveColiseum.processInfinite(ActiveColiseum.java:744) [ColiseumCraft-%5B1.4%5D.jar:?]
	at org.matecraft.kodaichi.coliseumcraft.ActiveColiseum.processGameplay(ActiveColiseum.java:564) [ColiseumCraft-%5B1.4%5D.jar:?]
	at org.matecraft.kodaichi.coliseumcraft.ColiseumCraft$2.run(ColiseumCraft.java:2840) [ColiseumCraft-%5B1.4%5D.jar:?]
	at org.bukkit.craftbukkit.v1_8_R3.scheduler.CraftTask.run(CraftTask.java:71) [classic.jar:git-Spigot-21fe707-e1ebe52]
	at org.bukkit.craftbukkit.v1_8_R3.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:350) [classic.jar:git-Spigot-21fe707-e1ebe52]
	at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:723) [classic.jar:git-Spigot-21fe707-e1ebe52]
	at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:374) [classic.jar:git-Spigot-21fe707-e1ebe52]
	at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:654) [classic.jar:git-Spigot-21fe707-e1ebe52]
	at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:557) [classic.jar:git-Spigot-21fe707-e1ebe52]
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_151]
Caused by: java.lang.NoClassDefFoundError: org/bukkit/entity/WitherSkeleton
	at io.hotmail.com.jacob_vejvoda.infernal_mobs.infernal_mobs.makeInfernal(infernal_mobs.java:372) ~[?:?]
	at io.hotmail.com.jacob_vejvoda.infernal_mobs.EventListener.onMobSpawn(EventListener.java:249) ~[?:?]
	at sun.reflect.GeneratedMethodAccessor121.invoke(Unknown Source) ~[?:?]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_151]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_151]
	at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:306) ~[classic.jar:git-Spigot-21fe707-e1ebe52]
	... 21 more