clipboard globalunlock windows

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Windows

clipboard

AddClipboardFormatListener(this->GetSafeHwnd());

if (IsClipboardFormatAvailable(CF_UNICODETEXT)) 
{
  if (::OpenClipboard(this->m_hWnd)) 
   {	
	HGLOBAL hg_clipdata = ::GetClipboardData(CF_UNICODETEXT);
	LPTSTR    lptstr; 			
	lptstr = (LPTSTR)::GlobalLock(hg_clipdata); 
	BOOL retv = ::GlobalUnlock(hg_clipdata); 
	BOOL ret = ::CloseClipboard(); 
    }
}

Hi im trying to get text from the clipboard. it works ok. in that lptstr gets the text fine. However if im copying from excel for example, i notice that occassionally if i select a cell color, i get an pop up "Error cannot empty clipboard". I notice that GlobalUnlock(hg_clipdata) returns 1. i cannot understand why as i have it straight after the only call to ::GlobalLock.