Problème substr syntaxe javascript

function departementret2(value) {  
 
if (document.forms["the_form"].villeb.value != ""){ 
 
document.forms["the_form"].depret2.value = document.forms["the_form"].villeb.substring(0,2); 
 
} 
} 

<form name="the_form" method="post" action="reservation.php?cat_chauff=2" onsubmit="return valider()"> 
    <tr> 
        <td><div align="right" class="margin3"><span class="bodyText style7">*</span><label for="objet">Ville :</label></div></td> 
        <td><input type="text" size="30"  maxlength="30" name="cpb" onkeyup="completionVilleb(this.value);" /></td>  
        </tr> 
        <tr> 
                <td><select name="villeb" id="villeb"> 
            <!--<option value="">Choix</option>--> 
        </select> 
        </td> 
        <td><input type="text" size="10"  maxlength="10" name="depret2" id="depret2" onkeyup="departementret2(this.value);" /></td>  
 
        </tr> 

function departementret2(value) {
  var szTmp = document.forms["the_form"].villeb.value;
  if ( szTmp != ""){
    document.forms["the_form"].depret2.value = szTmp.substring(0,2);
  }
}