required validator not working with selectOneListbox

[mfaine]

I have the following on my page:

<h:selectOneListbox id="document" value="#{sessionData.selectedDocumentId}"
required="true" size="5" style="width: 300px;"  >
            <f:selectItems id="documentItem"
value="#{sessionData.documentSelectItems}"/>
          </h:selectOneListbox>

... then

<h:commandButton action="#{sessionData.editAction}"
value="#{msg.common_edit}"
actionListener="#{sessionData.currentDocumentListen}" />

The editAction is allowed to execute even if selectedDocumentId is null? 
Actually it is the actionListener method that gets called regardless of the
value for selectedDocumentId.

Thanks,
-Mark