Bukkit

In my rankups file i had just this:

ranks:

a:0

(i already added a to GroupManager)

But when i start my server in my console it shows this:

[10:46:20 WARN]: java.lang.ClassCastException: java.util.ArrayList cannot be cast to org.bukkit.configuration.ConfigurationSection
[10:46:20 WARN]: at sh.okx.rankup.RankList.<init>(RankList.java:26)
[10:46:20 WARN]: at sh.okx.rankup.ranks.Rankups.<init>(Rankups.java:9)
[10:46:20 WARN]: at sh.okx.rankup.Rankup.refreshRanks(Rankup.java:258)
[10:46:20 WARN]: at sh.okx.rankup.Rankup.lambda$loadConfigs$3(Rankup.java:245)
[10:46:20 WARN]: at org.bukkit.craftbukkit.v1_8_R3.scheduler.CraftTask.run(CraftTask.java:71)
[10:46:20 WARN]: at org.bukkit.craftbukkit.v1_8_R3.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:350)
[10:46:20 WARN]: at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:723)
[10:46:20 WARN]: at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:374)
[10:46:20 WARN]: at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:654)
[10:46:20 WARN]: at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:557)
[10:46:20 WARN]: at java.lang.Thread.run(Unknown Source)
[10:46:20 ERROR]: [Rankup] Failed to load Rankup
[10:46:20 ERROR]: [Rankup] java.util.ArrayList cannot be cast to org.bukkit.configuration.ConfigurationSection