Bukkit

got this error can you tell me how to fix it:

[18:37:34] [pool-3-thread-18/WARN]: Exception in thread "pool-3-thread-18"
[18:37:34] [pool-3-thread-18/WARN]: org.apache.commons.lang.UnhandledException: Plugin CommandSigns v1.9.3.1 generated an exception while executing task 90
at org.bukkit.craftbukkit.v1_7_R3.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:56)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:722)
Caused by: java.lang.NoSuchMethodError: org.bukkit.Server.getOnlinePlayers()Ljava/util/Collection;
at org.zonedabone.commandsigns.thirdparty.Metrics.postPlugin(Metrics.java:488)
at org.zonedabone.commandsigns.thirdparty.Metrics.access$400(Metrics.java:71)
at org.zonedabone.commandsigns.thirdparty.Metrics$1.run(Metrics.java:641)
at org.bukkit.craftbukkit.v1_7_R3.scheduler.CraftTask.run(CraftTask.java:53)
at org.bukkit.craftbukkit.v1_7_R3.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:53)