Error

2012-11-11 22:39:43 [INFO] SkyDevil_ issued server command: /islandev purge 30
2012-11-11 22:39:46 [SEVERE] null
org.bukkit.command.CommandException: Unhandled exception executing command 'islandev' in plugin IslandWorld v1.3.1
	at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46)
	at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:185)
	at org.bukkit.craftbukkit.CraftServer.dispatchCommand(CraftServer.java:504)
	at net.minecraft.server.NetServerHandler.handleCommand(NetServerHandler.java:911)
	at net.minecraft.server.NetServerHandler.chat(NetServerHandler.java:824)
	at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:806)
	at net.minecraft.server.Packet3Chat.handle(Packet3Chat.java:44)
	at net.minecraft.server.NetworkManager.b(NetworkManager.java:282)
	at net.minecraft.server.NetServerHandler.d(NetServerHandler.java:111)
	at net.minecraft.server.ServerConnection.b(SourceFile:35)
	at net.minecraft.server.DedicatedServerConnection.b(SourceFile:30)
	at net.minecraft.server.MinecraftServer.r(MinecraftServer.java:578)
	at net.minecraft.server.DedicatedServer.r(DedicatedServer.java:215)
	at net.minecraft.server.MinecraftServer.q(MinecraftServer.java:495)
	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:428)
	at net.minecraft.server.ThreadServerApplication.run(SourceFile:818)
Caused by: java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
	at java.util.HashMap$EntryIterator.next(Unknown Source)
	at java.util.HashMap$EntryIterator.next(Unknown Source)
	at pl.islandworld.commands.IslandDev.commandPurge(IslandDev.java:237)
	at pl.islandworld.commands.IslandDev.onCommand(IslandDev.java:152)
	at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44)
	... 15 more