Thank u very much Miss Harper.

I think it's due to this code, and I don't really know a good way to rectify
this :

Jour
<html:select size="1" property="birthDay">
  <c:forEach var="aDay" begin="1" end="31">
    <option value="${aDay}"/>
    <c:out value="${aDay}"/>
  </c:forEach>
</html:select>

Mois
<html:select size="1" property="birthMonth">
  <c:forEach var="aMonth" begin="1" end="12">
    <option value="${aMonth}"/>
    <c:out value="${aMonth}"/>
  </c:forEach>
</html:select>


Thank u very much for your support.