Webbrowser问题，深受困扰，求指点迷津

小弟上周刚从VB转学Dephi，这两天研究webbrowser编程碰到一个奇怪的问题，深受困扰，希望大侠们指点迷津，万谢。以下模块是在XE2环境下调试的。
procedure HideTable(myweb:TWebBrowser);
var
  i:integer;
  mydoc:HtmlDocument;
  myelm:IHTMLElementCollection;
  mytab:HtmlTable;
begin
  mydoc:=myweb.Document as htmldocument;
  myelm:=mydoc.getElementsByTagName('table') as IHTMLElementCollection;
  //用for循环报错--Access Violation at address 00000000
  for i:=0 to myelm.length-1 do
  begin
  mytab:=myelm.item(i,varempty) as htmltable;
  (mytab.style as ihtmlstyle).display:='none';
  end;
//用以下语句逐条赋值没有报错
//mytab:=myelm.item(0,varempty) as htmltable;
//(mytab.style as ihtmlstyle).display:='none';
//mytab:=myelm.item(1,varempty) as htmltable;
//(mytab.style as ihtmlstyle).display:='none';
//mytab:=myelm.item(2,varempty) as htmltable;
//(mytab.style as ihtmlstyle).display:='none';
end;

很显然是myelm.length不对
mytab:=myelm.item(i,varempty) as htmltable;