Zombies v1.5.1 generated an exception

[14:30:18] [Server thread/WARN]: [Zombies] Task #2763 for Zombies v1.5.1 generated an exception
java.lang.IndexOutOfBoundsException: Index: 1, Size: 1
	at java.util.ArrayList.rangeCheck(ArrayList.java:653) ~[?:1.8.0_131]
	at java.util.ArrayList.get(ArrayList.java:429) ~[?:1.8.0_131]
	at com.matsg.zombies.game.Arena.spawnPlayers(Arena.java:167) ~[?:?]
	at com.matsg.zombies.game.Countdown$1.run(Countdown.java:72) ~[?:?]
	at org.bukkit.craftbukkit.v1_11_R1.scheduler.CraftTask.run(CraftTask.java:71) ~[Spigot.jar:git-Spigot-3fb9445-6e3cec8]
	at org.bukkit.craftbukkit.v1_11_R1.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:353) [Spigot.jar:git-Spigot-3fb9445-6e3cec8]
	at net.minecraft.server.v1_11_R1.MinecraftServer.D(MinecraftServer.java:738) [Spigot.jar:git-Spigot-3fb9445-6e3cec8]
	at net.minecraft.server.v1_11_R1.DedicatedServer.D(DedicatedServer.java:399) [Spigot.jar:git-Spigot-3fb9445-6e3cec8]
	at net.minecraft.server.v1_11_R1.MinecraftServer.C(MinecraftServer.java:678) [Spigot.jar:git-Spigot-3fb9445-6e3cec8]
	at net.minecraft.server.v1_11_R1.MinecraftServer.run(MinecraftServer.java:576) [Spigot.jar:git-Spigot-3fb9445-6e3cec8]
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_131]
[14:30:19] [Server thread/WARN]: [Zombies] Task #2763 for Zombies v1.5.1 generated an exception