Clear Form Elements - Multi Selection Does Not Work

Do you know why js does not reset this multi select box?
<select name="LOB" multiple="multiple" size="5" style="background:#fff3b3;width:150px" >
<option value="DP"
>DP</option>
<option value="DTV"
>DTV</option>
<option value="HSD"
>HSD</option>
<option value="PPV"
>PPV</option>
<option value="RF"
>RF</option>
<option value="VOD"
>VOD</option>
</select>

Code:
function clearForm(oForm) {
var elements = oForm.elements;
oForm.reset();
for(i=0; i<elements.length; i++) {
field_type = elements[i].type.toLowerCase();
switch(field_type) {
case "text":
case "password":
case "textarea":
case "hidden":

elements[i].value = "";
break;
case "radio":
case "checkbox":
if (elements[i].checked) {
elements[i].checked = false;
}
break;

case "select-one":
case "select-multi":
//alert(elements[i].value);
elements[i].selectedIndex = -1;
break;
default:
break;
}}}