Getting java.io.NotSerializableException when SessionObjectSize counter is enabled

Problem

When PMI (Performance Monitoring Infrastructure) is enabled with SessionObjectSize counter enabled, getting NotSerializableException. The SystemOut.log shows something similar to: [4/10/07 5:25:42:618 EDT] 00000091 SessionContex E Exception is: java.io.NotSerializableException: com.com1.dept.nls.model.someModel at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java(Compiled Code)) ... at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java(CompiledCode)) at com.ibm.ws.webcontainer.httpsession.SessionContextMBeanAdapter.getSessionsInfo(SessionContextMBeanAdapter.java(Compiled Code)) at com.ibm.ws.webcontainer.httpsession.SessionTrackingPMIApplicationData$1.getRuntimeInfo(SessionTrackingPMIApplicationData.java(Inlined Compiled Code)) ... at com.ibm.ws.pmi.server.modules.ServletSessionsModule$SessionObjSize.updateStatistic(ServletSessionsModule.java(Compiled Code)) at com.ibm.ws.pmi.server.data.SpdStatExternal.updateExternal(SpdStatExternal.java(Compiled Code)) at com.ibm.websphere.pmi.server.PmiAbstractModule.update(PmiAbstractModule.java(Compiled Code)) ... The error does not affect application functionality. Disabling SessionObjectSize counter makes the error stop occurring.