[07:44:33 WARN]: [DirtyArrows] Task #44 for DirtyArrows v3.0 generated an exception
java.lang.NoSuchMethodError: java.util.concurrent.ConcurrentHashMap.keySet()Ljava/util/concurrent/ConcurrentHashMap$KeySetView;
at nl.sugcube.dirtyarrows.ability.Iron.run(Iron.java:25) ~[?:?]
at org.bukkit.craftbukkit.v1_8_R2.scheduler.CraftTask.run(CraftTask.java:71) ~[spigot-1.8.3.jar:git-Spigot-870264a-0a645a2]
at org.bukkit.craftbukkit.v1_8_R2.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:350) [spigot-1.8.3.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.MinecraftServer.A(MinecraftServer.java:720) [spigot-1.8.3.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.DedicatedServer.A(DedicatedServer.java:368) [spigot-1.8.3.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.MinecraftServer.z(MinecraftServer.java:651) [spigot-1.8.3.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.MinecraftServer.run(MinecraftServer.java:554) [spigot-1.8.3.jar:git-Spigot-870264a-0a645a2]
at java.lang.Thread.run(Thread.java:745) [?:1.7.0_79]