大家帮忙看看，解决马上给分！

函数如下
function DaoActive(var DaoObject:OleVariant):Boolean;
begin
  Result:=False;
  try
    DaoObject:=GetActiveOleObject('DAO.DBEngine.36');
    Result:=True;
  except
    try
      DaoObject:=CreateOleObject('DAO.DBEngine.36');
      Result:=True;
    except
      DaoObject:=Null;     //这里
    end;
  end;
end;
错误提示 Undeclared identifier: 'Null'
Undeclared identifier: 'Unassigned'
怎么修改？