[rules-users] Facing Problem with this exception

Hi All, We are getting the following exceptions when drools is executing some rules. Exception: Caused by: java.lang.NullPointerException at org.drools.core.util.LeftTupleIndexHashTable.remove(LeftTupleIndexHashTable.java:251) at org.drools.reteoo.NotNode.retractLeftTuple(NotNode.java:226) at org.drools.reteoo.SingleLeftTupleSinkAdapter.doPropagateRetractLeftTuple(SingleLeftTupleSinkAdapter.java:217) at org.drools.reteoo.SingleLeftTupleSinkAdapter.propagateRetractRightTuple(SingleLeftTupleSinkAdapter.java:124) at org.drools.reteoo.JoinNode.retractRightTuple(JoinNode.java:151) at org.drools.reteoo.ObjectTypeNode.retractObject(ObjectTypeNode.java:232) at org.drools.reteoo.ReteooWorkingMemory$WorkingMemoryReteExpireAction.execute(ReteooWorkingMemory.java:412) at org.drools.common.AbstractWorkingMemory.executeQueuedActions(AbstractWorkingMemory.java:1471) ... 51 more Regards, Mallikarjun K