here is a sample call including the stack trace which describes the error:

$ twiddle.sh -s jnp://localhost:1099 get jboss.j2ee:service=EJB,jndiName=ejb/Dummy InvokeStats
13:51:56,120 ERROR [Twiddle] Exec failed
java.io.NotSerializableException: org.jboss.invocation.InvocationStatistics
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1081)
at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1375)
at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1347)
at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1290)
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1079)
...