I am trying to reset some values in a form using the a4j:actionParam tag. But it seams that null values never arrive in the target bean. The converter receives it correctly, returns null, but it is never set in the bean.

The target is to fill in the start and endDate for different predefined values (last week, last month etc). For the "This week" value, the endDate must be reset to null.

 <rich:menuItem value="Last week">
   <a4j:support event="onclick" reRender="criteriaStartCalendar,criteriaEndCalendar">
    <a4j:actionparam name="startDate" value="#{dateBean.lastWeekStart}" assignTo="#{targetBean.startDate}" />
    <a4j:actionparam name="endDate" value="#{dateBean.lastWeekEnd}" assignTo="#{targetBean.endDate}" />
   </a4j:support>
  </rich:menuItem>