sort name does not appears in column title link with default properties

I'm trying to use displaytag with external paging and sorting. I've implemented the PaginatedList interface and trying to display an instance of that implemented class through the following code:
 
<display:table requestURI="autoridades.do?sender=btPaginar" name="sessionScope.autoridades" class="padrao" sort="external" pagesize="10" id="a" cellpadding="0" cellspacing="0" >
 
<display:column title="Autoridade" sortable="true" sortName="Autoridade" headerClass="sortable">
${a.nome}
</display:column>
 
<display:column title="Cargo" sortable="true" sortName="Cargo" headerClass="sortable">
${a.cargo}
</display:column>
 
<display:column title="Aniversário" sortable="true" headerClass="sortable">
<c:if test="${a.diaAniversario > 0 && a.mesAniversario > 0}">
${a.diaAniversario}/${a.mesAniversario}
</c:if>
</display:column>
 
<display:column class="largura70" media="html" title="&nbsp" headerClass="sortable">
 
<a href="javascript: go(document.AutoridadesForm,'btVisualizar${a.codigo}')"><img src="./images/5a.gif" border="0" alt="Visualizar" /></a>
 
<a href="javascript: go(document.AutoridadesForm,'btEditar${a.codigo}')"><img src="./images/editar.gif" border="0" alt="Editar" /></a>
 
<a href="javascript: go(document.AutoridadesForm,'btExcluir${a.codigo}')"><img src="./images/cancelar.gif" border="0" alt="Excluir" /></a>
 
</display:column>
 
 
The table is displayed and paginations works fine, but sorting does not, as the link when a column head is clicked does not contain "sort=name", rather it shows a "sort=", an empty sort parameter, and without it it's not possible to the application to figure out what criterion should be used to sort the list.
 
No default property is modified.

Does anyone has a sugestion about what is going on?