does a4j:actionParam works on selectOneMenu?

[lmk]

hello,

I try to get label for selectItem on selectOneMenu using a4j:support and assign its value to a bean parameter

<h:selectOneMenu value="#{bean.user}" id="users" >
<f:selectItem itemLabel="All" itemValue="#{bean.zero}" />
<t:selectItems value="#{bean.userItems}" var="list"
itemLabel="#{list.lastname} #{list.firstname}"
itemValue="#{list.userId}" />
<a4j:support event="onchange">
<a4j:actionparam name="userName" value="#{list.lastname}" assignTo="#{bean.userName}" />
</a4j:support>
</h:selectOneMenu>

but the assigned value is already null.
can we user action:Param for this interest?
thanks!

[lmk]

sorry, it's not judicious wath am doing, it is normal that

#{list.lastnale}

lose value on the a4j:support..
there is another solution to send label value. I try to avoid to add another action for all selectOneMenu..