- ::SendMessage(GetWinHwnd(GetClientWindow()), WM_MDITILE, MDITILE_HORIZONTAL, 0);
+ wxASSERT_MSG( orient == wxHORIZONTAL || orient == wxVERTICAL,
+ _T("invalid orientation value") );
+
+ ::SendMessage(GetWinHwnd(GetClientWindow()), WM_MDITILE,
+ orient == wxHORIZONTAL ? MDITILE_HORIZONTAL
+ : MDITILE_VERTICAL, 0);