We just upgraded to 1.343 and are starting to use EC2 plugin for creating on-demand slaves, but Hudson keeps on spitting following error message in the log:

Feb 5, 2010 10:54:35 AM hudson.plugins.ec2.EC2RetentionStrategy check
INFO: Disconnecting i-aa302cc2
Feb 5, 2010 10:54:35 AM hudson.plugins.ec2.EC2Slave terminate
INFO: Terminated EC2 instance: i-aa302cc2
Feb 5, 2010 10:54:40 AM hudson.triggers.SafeTimerTask run
SEVERE: Timer task hudson.slaves.NodeProvisioner$NodeProvisionerInvoker@1089e14 failed
java.lang.NullPointerException
at hudson.plugins.ec2.EC2Cloud.provision(EC2Cloud.java:198)
at hudson.slaves.NodeProvisioner.update(NodeProvisioner.java:183)
at hudson.slaves.NodeProvisioner.access$000(NodeProvisioner.java:53)
at hudson.slaves.NodeProvisioner$NodeProvisionerInvoker.doRun(NodeProvisioner.java:217)
at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:54)
at java.util.TimerThread.mainLoop(Timer.java:512)
at java.util.TimerThread.run(Timer.java:462)
Feb 5, 2010 10:54:47 AM hudson.triggers.SafeTimerTask run
SEVERE: Timer task hudson.slaves.NodeProvisioner$NodeProvisionerInvoker@1089e14 failed
java.lang.NullPointerException
at hudson.plugins.ec2.EC2Cloud.provision(EC2Cloud.java:198)
at hudson.slaves.NodeProvisioner.update(NodeProvisioner.java:183)
at hudson.slaves.NodeProvisioner.access$000(NodeProvisioner.java:53)
at hudson.slaves.NodeProvisioner$NodeProvisionerInvoker.doRun(NodeProvisioner.java:217)
at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:54)
at java.util.TimerThread.mainLoop(Timer.java:512)
at java.util.TimerThread.run(Timer.java:462)
Feb 5, 2010 10:54:57 AM hudson.triggers.SafeTimerTask run
SEVERE: Timer task hudson.slaves.NodeProvisioner$NodeProvisionerInvoker@1089e14 failed
java.lang.NullPointerException
at hudson.plugins.ec2.EC2Cloud.provision(EC2Cloud.java:198)
at hudson.slaves.NodeProvisioner.update(NodeProvisioner.java:183)
at hudson.slaves.NodeProvisioner.access$000(NodeProvisioner.java:53)
at hudson.slaves.NodeProvisioner$NodeProvisionerInvoker.doRun(NodeProvisioner.java:217)
at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:54)
at java.util.TimerThread.mainLoop(Timer.java:512)
at java.util.TimerThread.run(Timer.java:462)
Feb 5, 2010 10:55:07 AM hudson.triggers.SafeTimerTask run

After stopping my cloud slave, I was unable to launch another one, logs just appending the above message every 10 secs.