options.length为null或不是对象

Question

你能帮我调试下面的代码吗？这是一个JavaScript交换两个列表框之间的列表项。仅在IE8中，它给出了错误：“options.length为null或不是对象”

<script>
function listbox_moveacross(sourceID,destID){
    var src=document.getElementById(sourceID);
    var dest=document.getElementById(destID);
    for(var count=0;count<src.options.length;count++){
        if(src.options[count].selected==true){
            var option=src.options[count];
            var newOption=document.createElement("option");
            newOption.value=option.value;newOption.text=option.text;
            newOption.selected=true;
            try{dest.add(newOption,null);
            src.remove(count,null);
            }
            catch(error){
                dest.add(newOption);
                src.remove(count);
            }
            count--;
        }
    }
}
</script>



<!-- Source listbox-->
<select name="list1" id="list1" multiple="multiple" style="height:120px;" ondblclick="listbox_moveacross('list1', 'selectedlist1')">
                        <option>Please Select...</option>
                        <option>Option 1</option>
                        <option>Option 2</option>
                        <option>Option 3</option>
                        <option>Option 4</option>
                        <option>Option 5</option>
                        <option>Option 6</option>
                    </select>

                        <a href="#" onclick="listbox_moveacross('list1', 'selectedlist1')">Add &gt;</a>
                        <br/>
                        <a href="#" onclick="listbox_moveacross('selectedlist1', 'list1')">&lt; Remove</a>

<!-- Destination listbox -->
                        <select id="selectedlist1" name="selectedlist1" multiple size="7" style="width:150px;" ondblclick="listbox_moveacross('selectedlist1', 'list1')"></select>