Mi sfugge qualcosa?

codice:
<script language="JavaScript" type="text/JavaScript">
<!--

function convalidaForm(Qform) 
{

if(document.getElementById('zn').selectedIndex == 0 ){
alert("Selezionare Zn.");
document.getElementById('zn').focus();
return false;
} 

// Abilita l'invio del FORM
return(true);
}
 
//-->
</script>

<form name="Qform" action="processaform.asp" method="post" onsubmit="return(convalidaForm(this));">


<select id="zn" name="zn" class="ctrl" onChange="window.document.location='form.asp?zn='+this.options[this.selectedIndex].value;">

<% ZNsel = request.querystring("zn") %>  

    <% if znsel <> "" then %>
    <option value="<%=znsel%>" selected="selected"><%=znsel%></option>
    <% else %>
    <option selected>Seleziona Zn</option>
      

<%
end if

If Not rec.EOF Then
Do While Not rec.EOF
ID_Zn = rec("id_zn")
%>
          <option value= "<% = Trim(rec("zn")) %>"><%  = Trim(rec("zn")) %></option>
<%
rec.MoveNext
Loop
End If
rec.close
%>

              </select>