为何找不到程序内嵌IE控件？

crow188

$handle = WinGetHandle("[class:Internet Explorer_Server;instance:5]")
MsgBox(0,"",$handle)
local $oIE = _IEAttach($handle, "hwnd")
If IsObj($oIE) Then
        MsgBox(0,"","找到窗口")
Else
        MsgBox(0,"","找不到窗口");
EndIf

$handle不为0，说明找到了内嵌的浏览器控件窗口，但是_IEAttach($handle, "hwnd")返回0
为什么？