documen.form.select_comcode.options为空或不是对象
<%///////////////////////////公司二级联动///////////////////////////////////
dim countc
Set rsgs = Server.CreateObject("ADODB.Recordset")
sql = "select unique comcode,comsname,uppercomcode from comcode where comtype[1] in ('1','2') order by 1"
set rsgs=conn.execute(sql)
//response.Write(rsgs(1))
%>
<script language = "JavaScript">
var onecountc;
onecountc=0; 
subcatc = new Array();
  <%
  countc = 0
  do while not rsgs.eof 
  %>
subcatc[<%=countc%>] = new Array("<%= trim(rsgs("comsname"))%>","<%= trim(rsgs("uppercomcode"))%>","<%= trim(rsgs("comcode"))%>");
  
  <%
  countc = countc + 1
rsgs.movenext
  loop
  rsgs.close
  %>
onecountc=<%=countc%>;

function changelocationc(locationidc) 
  { 
  document.form.select_depart.options[0]=new Option("请选择下级公司",""); 
  document.form.select_depart.length = 1;  

  var locationidc=locationidc; 
  var i; 
  for (i=0;i < onecountc; i++) 
  { 
  if (subcatc[i][1] == locationidc) 
  {  
  document.form.select_depart.options[document.form.select_depart.length] = new Option(subcatc[i][0], subcatc[i][2]); 

  }  
  } 
   
  } 
</script>

以下是表格中

<td colspan="2" valign="middle" ><span class="STYLE7"><font color="#000000">公司
  <select name="select_comcode" id="select_comcode" onChange="changelocationc(document.form.select_comcode.options[document.form.select_comcode.selectedIndex].value)">
  <option value="all" selected>全部</option>
  <% Set rs4 = Server.CreateObject("ADODB.Recordset")
sql="select comcode,comsname,comgrade from comcode where comgrade='4' and comtype[1]<>'3' "&whr_coma&" order by 1"
rs4.open sql,conn,1,3
For i = 1 to rs4.recordcount
If rs4.EOF Then Exit For
  response.write("<option value="&rs4("comcode")&">"&rs4("comsname")&"</option>")
rs4.movenext
next
rs4.close 
%>
  </select>

  </font>
<font color="#000000">
  <label></label>
  <select name="select_depart" id="select_depart">
  <option value="全部" selected>--请选择部门--</option>
  </select>

我想实现select_comcode和select_depart的二级联动，可是总是提示documen.form.select_comcode.options为空或不是对象这个错误，这个表单的id和name都是form，确认过没有错
求助是哪里不对，要怎么改？

------解决方案--------------------
自己一步一步调试。。慢慢的就会找到答案的