Error Heroes

> [11:46:01 ERROR]: Could not pass event CreatureSpawnEvent to Heroes v1.9.3-LEGACY-SNAPSHOT-6a1792b
org.bukkit.event.EventException
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:309) ~[beerspigot.jar:git-TacoSpigot-bf023c7d]
at co.aikar.timings.TimedEventExecutor.execute(TimedEventExecutor.java:78) ~[beerspigot.jar:git-TacoSpigot-bf023c7d]
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[beerspigot.jar:git-TacoSpigot-bf023c7d]
at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:508) [beerspigot.jar:git-TacoSpigot-bf023c7d]
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:493) [beerspigot.jar:git-TacoSpigot-bf023c7d]
at org.bukkit.craftbukkit.v1_8_R3.event.CraftEventFactory.callCreatureSpawnEvent(CraftEventFactory.java:316) [beerspigot.jar:git-TacoSpigot-bf023c7d]
at net.minecraft.server.v1_8_R3.World.addEntity(World.java:1041) [beerspigot.jar:git-TacoSpigot-bf023c7d]
at net.minecraft.server.v1_8_R3.SpawnerCreature.a(SpawnerCreature.java:191) [beerspigot.jar:git-TacoSpigot-bf023c7d]
at net.minecraft.server.v1_8_R3.WorldServer.doTick(WorldServer.java:227) [beerspigot.jar:git-TacoSpigot-bf023c7d]
at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:841) [beerspigot.jar:git-TacoSpigot-bf023c7d]
at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:422) [beerspigot.jar:git-TacoSpigot-bf023c7d]
at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:724) [beerspigot.jar:git-TacoSpigot-bf023c7d]
at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:627) [beerspigot.jar:git-TacoSpigot-bf023c7d]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_212]
Caused by: java.lang.NoClassDefFoundError: org/bukkit/attribute/Attribute
at com.herocraftonline.heroes.characters.Monster.applyHealthChanges(Monster.java:135) ~[Heroes.jar:?]
at com.herocraftonline.heroes.listeners.HEntityListener.onCreatureSpawnEarly(HEntityListener.java:378) ~[Heroes.jar:?]
at sun.reflect.GeneratedMethodAccessor88.invoke(Unknown Source) ~[?:?]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_212]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_212]
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:307) ~[beerspigot.jar:git-TacoSpigot-bf023c7d]
... 13 more