Minecraft /help error.

[23:15:51] [Server thread/ERROR]: Couldn't process command: 'help'
java.lang.IllegalArgumentException: Comparison method violates its general contr
act!
        at java.util.ComparableTimSort.mergeLo(Unknown Source) ~[?:1.8.0_31]
        at java.util.ComparableTimSort.mergeAt(Unknown Source) ~[?:1.8.0_31]
        at java.util.ComparableTimSort.mergeCollapse(Unknown Source) ~[?:1.8.0_3
1]
        at java.util.ComparableTimSort.sort(Unknown Source) ~[?:1.8.0_31]
        at java.util.Arrays.sort(Unknown Source) ~[?:1.8.0_31]
        at java.util.Arrays.sort(Unknown Source) ~[?:1.8.0_31]
        at java.util.ArrayList.sort(Unknown Source) ~[?:1.8.0_31]
        at java.util.Collections.sort(Unknown Source) ~[?:1.8.0_31]
        at net.minecraft.command.CommandHelp.func_71534_d(SourceFile:84) ~[au.cl
ass:?]
        at net.minecraft.command.CommandHelp.func_71515_b(SourceFile:38) ~[au.cl
ass:?]
        at net.minecraft.command.CommandHandler.func_71556_a(CommandHandler.java
:94) [z.class:?]
        at net.minecraft.network.NetHandlerPlayServer.func_147361_d(NetHandlerPl
ayServer.java:739) [nh.class:?]
        at net.minecraft.network.NetHandlerPlayServer.func_147354_a(NetHandlerPl
ayServer.java:718) [nh.class:?]
        at net.minecraft.network.play.client.C01PacketChatMessage.func_148833_a(
SourceFile:37) [ir.class:?]
        at net.minecraft.network.play.client.C01PacketChatMessage.func_148833_a(
SourceFile:9) [ir.class:?]
        at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java
:212) [ej.class:?]
        at net.minecraft.network.NetworkSystem.func_151269_c(NetworkSystem.java:
165) [nc.class:?]
        at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.jav
a:659) [MinecraftServer.class:?]
        at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(Dedicated
Server.java:334) [lt.class:?]
        at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.jav
a:547) [MinecraftServer.class:?]
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:427) [M
inecraftServer.class:?]
        at net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:685)
[li.class:?]
[23:16:08] [Server thread/INFO]: <REDACTED> "It said An unknown error occurred while atte
mpting to perform this command"