Error with sorting

[JavaError] java.lang.IllegalArgumentException: Comparison method violates its general contract!
calling <nfunction 'sort'> with no arguments
        at code_jam_C.rod:32
command sort()
        at code_jam_C.rod:32
caused by:
java.lang.IllegalArgumentException: Comparison method violates its general contract!
        at java.util.TimSort.mergeLo(TimSort.java:777)
        at java.util.TimSort.mergeAt(TimSort.java:514)
        at java.util.TimSort.mergeCollapse(TimSort.java:441)
        at java.util.TimSort.sort(TimSort.java:245)
        at java.util.Arrays.sort(Arrays.java:1512)
        at java.util.ArrayList.sort(ArrayList.java:1454)
        at org.kaivos.röda.commands.SortPopulator.lambda$populateSort$3(SortPopulator.java:104)
        at org.kaivos.röda.Interpreter.execWithoutErrorHandling(Interpreter.java:1034)
        at org.kaivos.röda.Interpreter.exec(Interpreter.java:950)
        at org.kaivos.röda.Interpreter.lambda$evalCommand$19(Interpreter.java:1218)
        at org.kaivos.röda.Interpreter.lambda$evalStatementCC$16(Interpreter.java:1091)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
        at java.util.concurrent.FutureTask.run(FutureTask.java:266)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:745)