ArrayIndexOutOfBoundsException on ClassConfigurator.get()

See the following stack trace:

[ERROR] 2016-07-09 14:26:05 [UDP-multicast receiver,shared=jgroups-shared-transport] - JGRP000030: null: failed handling incoming message: java.lang.ArrayIndexOutOfBoundsException: -1
java.lang.ArrayIndexOutOfBoundsException: -1
at org.jgroups.conf.ClassConfigurator.get(ClassConfigurator.java:161)
at org.jgroups.Message.readHeader(Message.java:936)
at org.jgroups.Message.readFrom(Message.java:811)
at org.jgroups.protocols.TP.handleSingleMessage(TP.java:1712)
at org.jgroups.protocols.TP.receive(TP.java:1654)
at org.jgroups.protocols.UDP$PacketReceiver.run(UDP.java:701)
at java.lang.Thread.run(Thread.java:745)

Stepping through the code with the debugger shows that the following line is failing at ClassConfigurator.get() with magic = -1 thus the java.lang.ArrayIndexOutOfBoundsException

return magic < 1024 ? magicMap[magic] : (Class)magicMapUser.get(Short.valueOf(magic));

See attached jgroups.xml for configuration. This showed up when upgrading to 3.6.10 from 3.6.8 which worked fine.