17:00:47 [WARNING] [Shopkeepers] Task #135 for Shopkeepers v1.13.3 generated an exception
java.util.ConcurrentModificationException
        at java.util.HashMap$HashIterator.remove(Unknown Source)
        at com.nisovin.shopkeepers.ShopkeepersPlugin$4.run(ShopkeepersPlugin.java:583)
        at org.bukkit.craftbukkit.v1_5_R3.scheduler.CraftTask.run(CraftTask.java:58)
        at org.bukkit.craftbukkit.v1_5_R3.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:344)
        at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:777)
        at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:320)
        at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:732)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:614)
        at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:573)