IncompatibleClassChangeError

[MardMallard]

On my server, when players attempt to challenge another player with right click or command, the following error appears and nothing happens in-game.

I am running Duel 0.1.2-b189 and WorldGuard 6.1

21.12 17:22:54 [Server] INFO ... 15 more
21.12 17:22:54 [Server] INFO at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) ~[spigot.jar:git-Spigot-db6de12-18fbb24]
21.12 17:22:54 [Server] INFO at com.me.tft_02.duel.commands.DuelCommand.onCommand(DuelCommand.java:42) ~[?:?]
21.12 17:22:54 [Server] INFO at com.me.tft_02.duel.commands.ChallengeCommand.onCommand(ChallengeCommand.java:38) ~[?:?]
21.12 17:22:54 [Server] INFO at com.me.tft_02.duel.util.player.DuelManager.canDuel(DuelManager.java:65) ~[?:?]
21.12 17:22:54 [Server] INFO at com.me.tft_02.duel.util.RegionUtils.canDuelHereWG(RegionUtils.java:22) ~[?:?]
21.12 17:22:54 [Server] INFO at com.me.tft_02.duel.util.RegionUtils.getRegion(RegionUtils.java:60) ~[?:?]
21.12 17:22:54 [Server] INFO Caused by: java.lang.IncompatibleClassChangeError: Found interface com.sk89q.worldguard.protection.ApplicableRegionSet, but class was expected
21.12 17:22:54 [Server] INFO at java.lang.Thread.run(Unknown Source) [?:1.7.0_72]
21.12 17:22:54 [Server] INFO at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:557) [spigot.jar:git-Spigot-db6de12-18fbb24]
21.12 17:22:54 [Server] INFO at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:654) [spigot.jar:git-Spigot-db6de12-18fbb24]
21.12 17:22:54 [Server] INFO at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:374) [spigot.jar:git-Spigot-db6de12-18fbb24]
21.12 17:22:54 [Server] INFO at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:715) [spigot.jar:git-Spigot-db6de12-18fbb24]
21.12 17:22:54 [Server] INFO at net.minecraft.server.v1_8_R3.SystemUtils.a(SourceFile:44) [spigot.jar:git-Spigot-db6de12-18fbb24]
21.12 17:22:54 [Server] INFO at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.7.0_72]
21.12 17:22:54 [Server] INFO at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.7.0_72]
21.12 17:22:54 [Server] INFO at net.minecraft.server.v1_8_R3.PlayerConnectionUtils$1.run(SourceFile:13) [spigot.jar:git-Spigot-db6de12-18fbb24]
21.12 17:22:54 [Server] INFO at net.minecraft.server.v1_8_R3.PacketPlayInChat.a(PacketPlayInChat.java:1) [spigot.jar:git-Spigot-db6de12-18fbb24]
21.12 17:22:54 [Server] INFO at net.minecraft.server.v1_8_R3.PacketPlayInChat.a(PacketPlayInChat.java:45) [spigot.jar:git-Spigot-db6de12-18fbb24]
21.12 17:22:54 [Server] INFO at net.minecraft.server.v1_8_R3.PlayerConnection.a(PlayerConnection.java:997) [spigot.jar:git-Spigot-db6de12-18fbb24]
21.12 17:22:54 [Server] INFO at net.minecraft.server.v1_8_R3.PlayerConnection.handleCommand(PlayerConnection.java:1162) [spigot.jar:git-Spigot-db6de12-18fbb24]
21.12 17:22:54 [Server] INFO at org.bukkit.craftbukkit.v1_8_R3.CraftServer.dispatchCommand(CraftServer.java:641) ~[spigot.jar:git-Spigot-db6de12-18fbb24]
21.12 17:22:54 [Server] INFO at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:141) ~[spigot.jar:git-Spigot-db6de12-18fbb24]
21.12 17:22:54 [Server] INFO at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46) ~[spigot.jar:git-Spigot-db6de12-18fbb24]
21.12 17:22:54 [Server] INFO org.bukkit.command.CommandException: Unhandled exception executing command 'duel' in plugin Duel v0.1.2-b189
21.12 17:22:54 [Server] INFO [17:22:54 ERROR]: null