sudden NullPointerException from ActiveMQSessionExecutor.wakeup

[easyl]

AMQ 4.1.2 embedded in Geronimo 2.1.3

setup in deployment plan with 
<config-property-setting name="ServerUrl">
vm://localhost?async=true&amp;jms.asyncDispatch=false&amp;jms.copyMessageOnSend=false&amp;jms.watchTopicAdvisories=false
</config-property-setting>
                        
there are 6 different Queues listened by MDBs which may send messages to
each other (almost with sync send/reply).

When the message traffic get huge, I would got such NPE

Exception in thread "VMTransport" java.lang.NullPointerException
        at
org.apache.activemq.ActiveMQSessionExecutor.wakeup(ActiveMQSessionExecutor.java:76)
        at
org.apache.activemq.ActiveMQSessionExecutor.execute(ActiveMQSessionExecutor.java:61)
        at 
org.apache.activemq.ActiveMQSession.dispatch(ActiveMQSession.java:1344)
        at
org.apache.activemq.ActiveMQConnection.onCommand(ActiveMQConnection.java:1485)
        at
org.apache.activemq.transport.ResponseCorrelator.onCommand(ResponseCorrelator.java:95)
        at
org.apache.activemq.transport.TransportFilter.onCommand(TransportFilter.java:65)
        at
org.apache.activemq.transport.vm.VMTransport.iterate(VMTransport.java:201)
        at
org.apache.activemq.thread.PooledTaskRunner.runTask(PooledTaskRunner.java:120)
        at
org.apache.activemq.thread.PooledTaskRunner.access$100(PooledTaskRunner.java:26)
        at
org.apache.activemq.thread.PooledTaskRunner$1.run(PooledTaskRunner.java:47)
        at
edu.emory.mathcs.backport.java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:665)
        at
edu.emory.mathcs.backport.java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:690)
        at java.lang.Thread.run(Thread.java:619)


then some messages will be stalled in queue (checked with jconsole).

what does the NPE mean? 
how can I avoid the problem?

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