set category and typology on a working copy

Hi,
i'm trying to set category and typology on a working copy but an com.ibm.team.repository.common.internal.ImmutablePropertyException has been raised during the execution of the code.

This is the piece of code that raises the exception:

            ServiceManagementAutomaticFieldsManagementBean bean = map.get(macroCategoriaLiteral + SEPARATOR + categoriaLiteral + SEPARATOR + sottoCategoriaLiteral);       
            if (bean != null) {
                IWorkItemType workItemType = null;
                for (IWorkItemType iWorkItemType : findWorkItemTypes) {
                    if (iWorkItemType.getDisplayName().equalsIgnoreCase(bean.getTypology())) {
                        workItemType = iWorkItemType;
                        break;
                    }
                }
                //serviceManager.getWiService().findCategoryByNamePath(newStateWi.getProjectArea(), namePath, monitor)
                ICategory correctCategory = null;
                for (ICategory iCategory : categories) {
                    if (iCategory.getName().equalsIgnoreCase(bean.getCompetenceCenter())) {
                        correctCategory = iCategory;
                        break;
                    }
                }
               
                IWorkItemType findWorkItemType = serviceManager.getWiService().findWorkItemType(newStateWi.getProjectArea(), newStateWi.getWorkItemType(), null);
                IWorkItem workingCopy = (IWorkItem)serviceManager.getWiService().findWorkItemById(newStateWi.getId(), IWorkItem.DEFAULT_PROFILE, null).getWorkingCopy();
                boolean test = workingCopy.isWorkingCopy();
                serviceManager.getWiService().updateWorkItemType(workingCopy, workItemType, findWorkItemType, null);               
                workingCopy.setCategory(correctCategory);
                serviceManager.getWiService().saveWorkItem2(workingCopy, saveParameter.getNewReferences(), null);
            }


During the updateWorkItemType() and setCategory() invocation the exception is raised.
Any idea?

Thanks.