Facing IOException while storing workItem in jBPM5 database tables

Issue

• Facing IOException while storing workItem in jBPM5 database tables. To give a background , custom Java objects are used as a output parameter in jBPM5 process workItem. Apart from that inside the custom Java objects (e.g. CurrentStation) some objects from another custom Java classes (e.g. AllowedTrainTypes) are stored before passing them as parameters to complete the workitem.
  e.g.

@Override
    public void executeWorkItem(WorkItem workitem, WorkItemManager manager) {
        CurrentStation cstation = new CurrentStation("Howrah", "Kolkata");
        AllowedTrainTypes attype = new AllowedTrainTypes("SuperFast");
                cstation.setAllowedTrainTypes(attype);
                ...
            Map<String,Object> params = new HashMap<String,Object>();
                params.put("firstParam",cstation);
            manager.completeWorkItem(workitem.getId(),params);
       ...

• What is the reason behind facing the following exception and how to fix it?

2014-02-11 12:16:52,974 ERROR [org.drools.persistence.SingleSessionCommandService] (http-127.0.0.1-8080-35) Could not commit session
java.lang.IllegalArgumentException: IOException while storing workItem 414: com.sample.CurrentStation
    at org.drools.persistence.info.WorkItemInfo.update(WorkItemInfo.java:140)
    at org.drools.persistence.jpa.processinstance.JPAWorkItemManager.internalExecuteWorkItem(JPAWorkItemManager.java:46)
    at org.jbpm.workflow.instance.node.WorkItemNodeInstance.internalTrigger(WorkItemNodeInstance.java:107)
    at org.jbpm.workflow.instance.impl.NodeInstanceImpl.trigger(NodeInstanceImpl.java:122)
    at org.jbpm.workflow.instance.impl.NodeInstanceImpl.triggerNodeInstance(NodeInstanceImpl.java:210)
    at org.jbpm.workflow.instance.impl.NodeInstanceImpl.triggerCompleted(NodeInstanceImpl.java:169)
    at org.jbpm.workflow.instance.impl.ExtendedNodeInstanceImpl.triggerCompleted(ExtendedNodeInstanceImpl.java:47)
    at org.jbpm.workflow.instance.node.StateBasedNodeInstance.triggerCompleted(StateBasedNodeInstance.java:162)
    at org.jbpm.workflow.instance.node.StateBasedNodeInstance.triggerCompleted(StateBasedNodeInstance.java:143)
    at org.jbpm.workflow.instance.node.WorkItemNodeInstance.triggerCompleted(WorkItemNodeInstance.java:247)
    at org.jbpm.workflow.instance.node.WorkItemNodeInstance.workItemCompleted(WorkItemNodeInstance.java:309)
    at org.jbpm.workflow.instance.node.WorkItemNodeInstance.signalEvent(WorkItemNodeInstance.java:285)
    at org.jbpm.workflow.instance.impl.WorkflowProcessInstanceImpl.signalEvent(WorkflowProcessInstanceImpl.java:338)
    at org.drools.persistence.jpa.processinstance.JPAWorkItemManager.completeWorkItem(JPAWorkItemManager.java:121)
    at com.sample.Test.executeWorkItem(Test.java:15)
    at org.drools.persistence.jpa.processinstance.JPAWorkItemManager.internalExecuteWorkItem(JPAWorkItemManager.java:55)
    at org.jbpm.workflow.instance.node.WorkItemNodeInstance.internalTrigger(WorkItemNodeInstance.java:107)
    ...