[18:10:00 ERROR]: null
org.bukkit.command.CommandException: Unhandled exception executing command 'flyheads' in plugin FlyingHeads v1.0
        at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46) ~[craftbukkit.jar:git-Spigot-1211]
        at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:196) ~[craftbukkit.jar:git-Spigot-1211]
        at org.bukkit.craftbukkit.v1_7_R1.CraftServer.dispatchCommand(CraftServer.java:546) ~[craftbukkit.jar:git-Spigot-1211]
        at net.minecraft.server.v1_7_R1.PlayerConnection.handleCommand(PlayerConnection.java:940) [craftbukkit.jar:git-Spigot-1211]
        at net.minecraft.server.v1_7_R1.PlayerConnection.a(PlayerConnection.java:815) [craftbukkit.jar:git-Spigot-1211]
        at net.minecraft.server.v1_7_R1.PacketPlayInChat.a(PacketPlayInChat.java:28) [craftbukkit.jar:git-Spigot-1211]
        at net.minecraft.server.v1_7_R1.PacketPlayInChat.handle(PacketPlayInChat.java:65) [craftbukkit.jar:git-Spigot-1211]
        at net.minecraft.server.v1_7_R1.NetworkManager.a(NetworkManager.java:147) [craftbukkit.jar:git-Spigot-1211]
        at net.minecraft.server.v1_7_R1.ServerConnection.c(SourceFile:134) [craftbukkit.jar:git-Spigot-1211]
        at net.minecraft.server.v1_7_R1.MinecraftServer.u(MinecraftServer.java:657) [craftbukkit.jar:git-Spigot-1211]
        at net.minecraft.server.v1_7_R1.DedicatedServer.u(DedicatedServer.java:259) [craftbukkit.jar:git-Spigot-1211]
        at net.minecraft.server.v1_7_R1.MinecraftServer.t(MinecraftServer.java:540) [craftbukkit.jar:git-Spigot-1211]
        at net.minecraft.server.v1_7_R1.MinecraftServer.run(MinecraftServer.java:446) [craftbukkit.jar:git-Spigot-1211]
        at net.minecraft.server.v1_7_R1.ThreadServerApplication.run(SourceFile:617) [craftbukkit.jar:git-Spigot-1211]
Caused by: java.lang.NumberFormatException: For input string: "stone"
        at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65) ~[?:1.7.0_45]
        at java.lang.Integer.parseInt(Integer.java:492) ~[?:1.7.0_45]
        at java.lang.Integer.parseInt(Integer.java:527) ~[?:1.7.0_45]
        at me.fred12i12i.FlyingHeads.FlyingHeads.onCommand(FlyingHeads.java:103) ~[?:?]
        at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) ~[craftbukkit.jar:git-Spigot-1211]
        ... 13 more