HorseKeep - Bug

[13:51:33] [Server thread/INFO]: xAgigx issued server command: /h summon 118
[13:51:33] [Server thread/ERROR]: null
org.bukkit.command.CommandException: Unhandled exception executing command 'h' in plugin HorseKeep v0.3.0
	at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46) ~[spigot.jar:git-Spigot-c46583f-e6f9fd4]
	at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:141) ~[spigot.jar:git-Spigot-c46583f-e6f9fd4]
	at org.bukkit.craftbukkit.v1_8_R1.CraftServer.dispatchCommand(CraftServer.java:645) ~[spigot.jar:git-Spigot-c46583f-e6f9fd4]
	at net.minecraft.server.v1_8_R1.PlayerConnection.handleCommand(PlayerConnection.java:1115) [spigot.jar:git-Spigot-c46583f-e6f9fd4]
	at net.minecraft.server.v1_8_R1.PlayerConnection.a(PlayerConnection.java:950) [spigot.jar:git-Spigot-c46583f-e6f9fd4]
	at net.minecraft.server.v1_8_R1.PacketPlayInChat.a(PacketPlayInChat.java:26) [spigot.jar:git-Spigot-c46583f-e6f9fd4]
	at net.minecraft.server.v1_8_R1.PacketPlayInChat.a(PacketPlayInChat.java:53) [spigot.jar:git-Spigot-c46583f-e6f9fd4]
	at net.minecraft.server.v1_8_R1.PacketHandleTask.run(SourceFile:13) [spigot.jar:git-Spigot-c46583f-e6f9fd4]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471) [?:1.7.0_76]
	at java.util.concurrent.FutureTask.run(FutureTask.java:262) [?:1.7.0_76]
	at net.minecraft.server.v1_8_R1.MinecraftServer.z(MinecraftServer.java:683) [spigot.jar:git-Spigot-c46583f-e6f9fd4]
	at net.minecraft.server.v1_8_R1.DedicatedServer.z(DedicatedServer.java:316) [spigot.jar:git-Spigot-c46583f-e6f9fd4]
	at net.minecraft.server.v1_8_R1.MinecraftServer.y(MinecraftServer.java:623) [spigot.jar:git-Spigot-c46583f-e6f9fd4]
	at net.minecraft.server.v1_8_R1.MinecraftServer.run(MinecraftServer.java:526) [spigot.jar:git-Spigot-c46583f-e6f9fd4]
	at java.lang.Thread.run(Thread.java:745) [?:1.7.0_76]
Caused by: java.lang.ClassCastException: [Lorg.bukkit.inventory.ItemStack; cannot be cast to java.util.List
	at main.java.com.gmail.falistos.HorseKeep.HorseManager.summon(HorseManager.java:433) ~[?:?]
	at main.java.com.gmail.falistos.HorseKeep.commands.CommandSummon.<init>(CommandSummon.java:50) ~[?:?]
	at main.java.com.gmail.falistos.HorseKeep.HorseKeep.onCommand(HorseKeep.java:172) ~[?:?]
	at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) ~[spigot.jar:git-Spigot-c46583f-e6f9fd4]
	... 14 more