A depressing problem with ListBox!

A depressing problem with ListBox! I wasted more than an hour! Very depressed ......

Generated Code For example:
<Select name = "inc_client_accessmanage1: listbox1" size = "5" multiple = "multiple" id = "inc_client_accessmanage#listbox1" class = "s_verdana" style = "width: 240px;">
<Option value = "-1"> & lt; ---- new group name ---- & gt; </option>
<Option value = "89"> apple_002 </option>
<Option value = "-1"> & lt; ---- New group2 ---- & gt; </option>
<Option value = "6"> country </option>
<Option value = "-1"> & lt; ---- China office ---- & gt; </option>
<Option value = "88"> apple_001 </option>
<Option value = "188"> tempuser2 </option>
<Option value = "189"> country. Wu </option>
<Option value = "190"> country. wu2 </option> </SELECT>
After debugging, it is found that if you select the options of value =-1, the value obtained by using selectedindex will be 0!
I was puzzled and later found that all the options with the same value would be the selectedindex of the first option, which is unreasonable, later I had to change it to this !!!!! Depressing things .....
<Select name = "inc_client_accessmanage1: listbox1" size = "5" multiple = "multiple" id = "inc_client_accessmanage#listbox1" class = "s_verdana" style = "width: 240px;">
<Option value = "-1"> & lt; ---- new group name ---- & gt; </option>
<Option value = "89"> apple_002 </option>
<Option value = "-2"> & lt; ---- New group2 ---- & gt; </option>
<Option value = "6"> country </option>
<Option value = "-3"> & lt; ---- China office ---- & gt; </option>
<Option value = "88"> apple_001 </option>
<Option value = "188"> tempuser2 </option>
<Option value = "189"> country. Wu </option>
<Option value = "190"> country. wu2 </option> </SELECT>