SmartItems Error

01:32 PM [WARNING] [Smart_Items] Task #163 for Smart_Items v6.4 generated an exception
java.lang.IncompatibleClassChangeError: Found interface com.sk89q.worldguard.protection.ApplicableRegionSet, but class was expected
	at info.nothingspecial.Smart_Items.tools.canIbuild(tools.java:345)
	at info.nothingspecial.Smart_Items.tools.useBoneMealon(tools.java:246)
	at info.nothingspecial.Smart_Items.Smart_Items.run(Smart_Items.java:557)
	at org.bukkit.craftbukkit.v1_7_R4.scheduler.CraftTask.run(CraftTask.java:67)
	at org.bukkit.craftbukkit.v1_7_R4.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:345)
	at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:863)
	at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:422)
	at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:803)
	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:663)
	at java.lang.Thread.run(Unknown Source)