为什么在于子窗口MessageBoxTimeout无效

Private Declare Function MessageBoxTimeout Lib "user32" Alias "MessageBoxTimeoutA" _
    (ByVal hwnd As Long, ByVal lpText As String, _
    ByVal lpCaption As String, ByVal wType As Long, _
    ByVal wlange As Long, ByVal dwTimeout As Long) As Long

MessageBoxTimeout me.hwnd, "此任务已完成。", , "tips", vbInformation, 0, 1000

以上为自动关闭消息对话框，但是在子窗口中无效，希望有高手解惑。