Not able to add task reassignment through TaskEventListener.

WARN  [org.jbpm.services.task.persistence.TaskTransactionInterceptor] (default task-8) Could not commit session: java.lang.UnsupportedOperationException
 at java.util.AbstractList.add(AbstractList.java:148)
 at java.util.AbstractList.add(AbstractList.java:108)
 at org.hibernate.collection.internal.PersistentBag.add(PersistentBag.java:411)
 at org.jbpm.kie.services.impl.admin.commands.ScheduleTaskDeadlineCommand.execute(ScheduleTaskDeadlineCommand.java:69)  

import org.jbpm.services.api.admin.UserTaskAdminService;
import org.jbpm.services.api.service.ServiceRegistry;    
    public void afterTaskActivatedEvent(TaskEvent event) {
        // TODO Auto-generated method stub
UserTaskAdminService adminTaskService = (UserTaskAdminService) ServiceRegistry.get().service(ServiceRegistry.USER_TASK_ADMIN_SERVICE);

adminTaskService.reassignWhenNotStarted(event.getTask().getId(), "60s", TaskModelProvider.getFactory().newUser("user1"));
    }