SubCalendarSummary does not implement java.io.Serializable

java.lang.IllegalArgumentException: Resource is not serializable: Key=353e3df5-351b-486a-a5ef-afb5df1298ce, Value=com.atlassian.confluence.extra.calendar3.model.SubCalendarSummary@2d434736
        at com.tangosol.coherence.component.util.daemon.queueProcessor.service.ReplicatedCache.updateResource(ReplicatedCache.CDB:7)
        at com.tangosol.coherence.component.util.CacheHandler.put(CacheHandler.CDB:11)
        at com.tangosol.coherence.component.util.CacheHandler.put(CacheHandler.CDB:1)
        at com.tangosol.coherence.component.util.SafeNamedCache.put(SafeNamedCache.CDB:1)
        at com.atlassian.confluence.cache.tangosol.TangosolCache.put(TangosolCache.java:41)
        at com.atlassian.confluence.extra.calendar3.calendarstore.CalendarDataStoreCachingDecorator.getSubCalendarSummary(CalendarDataStoreCachingDecorator.java:156)
        at com.atlassian.confluence.extra.calendar3.calendarstore.DelegatingCalendarDataStore.getSubCalendarSummary(DelegatingCalendarDataStore.java:115)
        at com.atlassian.confluence.extra.calendar3.DefaultCalendarManager.getSubCalendarSummary(DefaultCalendarManager.java:172)