Bukkit update 1.6.4 error

Would anybody anybody have any idea what this means? These messages don't usually show up in my console, and now since they've shown up I can't connect to my server. Could it mean that this particular plugin isn't up to date? Or is it more than that?


15:08:40 [INFO] [SafeCreeeper] Enabling SafeCreeper v1.5.1.1
15:08:40 [INFO] [SafeCreeper] Safe Creeper API started!
15:08:40 [INFO] [SafeCreeper] Safe Creeper API enabled!
15:08:40 [INFO] [SafeCreeper] Loading global config...
15:08:40 [INFO] [SafeCreeper] Global config loaded, took 36 ms!
15:08:40 [INFO] [SafeCreeper] Loading world configs...
15:08:40 [INFO] [SafeCreeper] 2 world configs loaded, took 4 ms!
15:08:40 [INFO] [SafeCreeper] Checking for updates...
15:08:40 [INFO] [SafeCreeper] [ERROR] Error occured while checking for updates!
15:08:40 [INFO] [SafeCreeper] New Safe Creeper version available: v1.5.2
15:08:40 [SEVERE] Error occurred while enabling SafeCreeper v1.5.1.1 (Is it up t
o date?)
java.lang.NoSuchMethodError: org.json.JSONObject.isString(Ljava/lang/String;)Z
at com.timvisee.safecreeper.util.SCUpdateChecker.isRequiredBukkitVersion
Set(SCUpdateChecker.java:197)
at com.timvisee.safecreeper.util.SCUpdateChecker.isNewVersionCompatibleW
ithCurrentBukkit(SCUpdateChecker.java:167)
at com.timvisee.safecreeper.SafeCreeper.onEnable(SafeCreeper.java:132)
at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:217)
at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader
.java:457)
at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManage
r.java:381)
at org.bukkit.craftbukkit.v1_6_R2.CraftServer.loadPlugin(CraftServer.jav
a:282)
at org.bukkit.craftbukkit.v1_6_R2.CraftServer.enablePlugins(CraftServer.
java:264)
at org.bukkit.craftbukkit.v1_6_R2.CraftServer.reload(CraftServer.java:60
8)
at org.bukkit.Bukkit.reload(Bukkit.java:277)
at org.bukkit.command.defaults.ReloadCommand.execute(ReloadCommand.java:
23)
at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:19
2)
at org.bukkit.craftbukkit.v1_6_R2.CraftServer.dispatchCommand(CraftServe
r.java:523)
at org.bukkit.craftbukkit.v1_6_R2.CraftServer.dispatchServerCommand(Craf
tServer.java:512)
at net.minecraft.server.v1_6_R2.DedicatedServer.ar(DedicatedServer.java:
262)
at net.minecraft.server.v1_6_R2.DedicatedServer.t(DedicatedServer.java:2
27)
at net.minecraft.server.v1_6_R2.MinecraftServer.s(MinecraftServer.java:4
86)
at net.minecraft.server.v1_6_R2.MinecraftServer.run(MinecraftServer.java
:419)
at net.minecraft.server.v1_6_R2.ThreadServerApplication.run(SourceFile:5
82)