Null key not allowed exception in iqhandler

I have installed openfire in 3 nodes and added all there in cluster using hazelcast plugin. I got this below error which am not sure why it is occuring. This exception was not there when we had single node. Please help me to identify the cause and rectify this issue.

2015.04.14 11:15:32 org.jivesoftware.openfire.handler.IQHandler - Internal server error

java.lang.NullPointerException: Null key is not allowed!

*** at***

*** at com.jivesoftware.util.cache.ClusteredCache.get(***

*** at org.jivesoftware.util.cache.CacheWrapper.get(***

*** at com.cryptall.service.impl.OTPManagementImpl.validateOTP( 46)***

*** at com.cryptall.iqhandler.IQOtpHandler.handleIQ(***

*** at org.jivesoftware.openfire.handler.IQHandler.process(***

*** at org.jivesoftware.openfire.IQRouter.handle(***

*** at org.jivesoftware.openfire.IQRouter.route(***

*** at org.jivesoftware.openfire.spi.PacketRouterImpl.route(***

*** at***

*** at .java:93)***

*** at***

*** at***

*** at org.jivesoftware.openfire.nio.ConnectionHandler.messageReceived(ConnectionHandl***

Thanks in Advance.