I got the following error while running jarjar:
[java] jarjar: cannot find index 0 in {6=<clinit>, 7=<init>, 8=@, 9=Code, 10=Exceptions, 11=LineNumberTable,
12=Object.java, 13=SourceFile, 14=append, 15=clone, 16=equals, 17=finalize, 18=getClass, 19=getName, 20=hashCode,
21=java/lang/Class, 22=java/lang/CloneNotSupportedException, 23=java/lang/IllegalArgumentException,
4=java/lang/Integer, 25=java/lang/InterruptedException, 26=java/lang/Object, 27=java/lang/StringBuffer,
28=java/lang/Throwable, 29=nanosecond timeout value out of range, 30=notify, 31=notifyAll, 32=registerNatives,
33=timeout value is negative, 34=toHexString, 35=toString, 36=wait, 37=21, 38=22, 39=23, 40=24, 41=25, 42=26,
43=27, 44=28, 45=()I, 46=()V, 47=(J)V, 48=(JI)V, 49=()Ljava/lang/Class;, 50=()Ljava/lang/Object;,
51=(Ljava/lang/Object;)Z, 52=()Ljava/lang/String;, 53=(I)Ljava/lang/String;, 54=(Ljava/lang/String;)V,
55=(Ljava/lang/String;)Ljava/lang/StringBuffer;}
</init></clinit>
The solution was to remove the jre library rt.jar from the path analysed by jarjar.