更新面板抛出javascript错误：“ Microsoft JScript运行时错误：找不到成员”

Microsoft JScript runtime error: Member not found

<script type=\"text/javascript\">
//<![CDATA[
var theForm = document.forms[\'ctl01\'];
if (!theForm) {
    theForm = document.ctl01;
}
function __doPostBack(eventTarget, eventArgument) {
    if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
        theForm.__EVENTTARGET.value = eventTarget;
        theForm.__EVENTARGUMENT.value = eventArgument;
        theForm.submit();
    }
}
//]]>
</script>

    <asp:ScriptManager ID=\"manager\" runat=\"server\" ></asp:ScriptManager>
    <asp:UpdatePanel ID=\"platformOutputTypes\" runat=\"server\" UpdateMode=\"Always\" >
        <ContentTemplate >
            <p>
                <label>Platform</label>
                <asp:DropDownList ID=\"platform\" AutoPostBack=\"true\" runat=\"server\" onselectedindexchanged=\"PlatformSelectedIndexChanged\" ></asp:DropDownList>
            </p>

            <asp:CheckBoxList TextAlign=\"Left\" ID=\"reportOutputTypes\" runat=\"server\" />
        </ContentTemplate>
   </asp:UpdatePanel>