列出Webbrowser1里所有的 frames 里的 hyper link，到第三层frame就出错了（拒绝访问），why ？（内有代码(c)）(100分)

procedure ListAllLinks(Doc_: variant ; Stringlist_ :TStringlist );
var i : integer;
Doc: variant;
begin
Doc := Doc_;

if doc.frames.length = 0 then
begin

For i := 0 To Doc.links.length - 1 do
Stringlist_.Add(Doc.links.item(i).toString ); //Doc.links(i).outerText & " " & Doc.links(i));

end Else
begin
For i := 0 To Doc.frames.length - 1 do
begin
Doc.Frames.item(i);
ListAllLinks(Doc.Frames.item(i).document,stringlist_);// form1.WebBrowser1.OleObject.Document.frames.item(i).document , stringlist_);
end;
end;
End;

三层frame的网站之一： http://winsock.xilubbs.com/

错误对话框：
---------------------------
Debugger Exception Notification
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Project Project1.exe raised exception class EOleException with message '拒绝访问。'. Process stopped. Use Step or Run to continue.
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OK Help
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注：网页已经完全打开完毕