这段Webbrowser 的框架代码哪里出了问题(图)

procedure TForm1.Button5Click(Sender: TObject);
var
doc,FrameDoc:IHtmlDocument2;
FramesColl:IHtmlFramesCollection2;
win:IHtmlwindow2;
vFrame,button1,ret:oleVariant;
x:ihtmlinputelement;
begin
doc:=Webbrowser1.Document as IHtmlDocument2;
FramesColl:=(doc.get_frames as IHtmlFramesCollection2);
TVariantArg(vFrame).vt:=VT_UINT;
TVariantArg(vFrame).iVal:=0;
ret:=FramesColl.item(vFrame);
TVariantArg(ret).pdispVal.QueryInterface(IID_IHTMLWindow2,win);
FrameDoc:=(win.document as IHtmlDocument2);
x:=(FrameDoc.all.Item('x',0) as ihtmlinputelement);
x.value:='1000';
button1:=FrameDoc.all.item('submit',0);
button1.click;
end;
提示Access violation at address 8b55ff8b....