Tries to pass the null to ConcurrentMap.keySet().contains(), which throws NPE:

2008-02-28 17:43:06,098 ERROR [org.jboss.messaging.util.ExceptionUtil] org.jboss.messaging.core.jmx.MessagingPostOfficeService@1a0d916 startService
org.jgroups.ChannelException: failed to start protocol stack
at org.jgroups.JChannel.startStack(JChannel.java:1440)
at org.jgroups.JChannel.connect(JChannel.java:362)
at org.jboss.messaging.core.impl.postoffice.GroupMember.start(GroupMember.java:142)
....
Caused by: java.lang.NullPointerException
at java.util.concurrent.ConcurrentHashMap.hash(ConcurrentHashMap.java:157)
at java.util.concurrent.ConcurrentHashMap.containsKey(ConcurrentHashMap.java:745)
at java.util.concurrent.ConcurrentHashMap$KeySet.contains(ConcurrentHashMap.java:1215)
at org.jgroups.stack.Configurator.startProtocolStack(Configurator.java:85)
at org.jgroups.stack.ProtocolStack.startStack(ProtocolStack.java:341)
at org.jgroups.JChannel.startStack(JChannel.java:1437)
... 59 more

In JChannel.startStack(String cluster_name) there's this which implies a null cluster_name is meant to work:

if(cluster_name == null) {
if(log.isDebugEnabled()) log.debug("cluster_name is null, assuming unicast channel");
}

If it isn't meant to work any longer, then an IllegalArgumentException is better than NPE.