idhttp多线程访问问题，请各位高人帮忙看看

type 部分
 TMMThread =class(TThread)
    tthreadid:integer;
    tthreadtype:integer;
    urlStr :String;
    ttpostdata :TStrings;
    ttresponses : TStringStream;
    constructor Create(ttid:integer;typeflag:integer;url:String;postdate:TStrings;responses : TStringStream);
    procedure Execute; override;
  end;{ TMMThread }constructor TMMThread.create(ttid:integer;typeflag:integer;url:String;postdate:TStrings;responses : TStringStream);
begin
  inherited Create(True);
  FreeOnTerminate:=True;
  Suspended:=false;
  tthreadid:=ttid;
  tthreadtype:=typeflag;
  urlStr :=url;
  ttpostdata :=postdate;
  ttresponses := responses;end;
procedure TMMThread.Execute;
var
 thIdHTTP: TIdHTTP;
begin
  try
    thIdHTTP := TIdHTTP.Create(nil);
    if tthreadtype =1 then
    begin
      thIdHTTP.HandleRedirects := True;
      thIdHTTP.Request.ContentType := 'application/x-www-form-urlencoded' ;
      thIdHTTP.Post(urlStr,ttpostdata,ttresponses);
    end
    else if tthreadtype =2 then
    begin
      thIdHTTP.get(urlStr,ttresponses);
    end;
  finally
    thIdHTTP.free;
  end;
end;var
 Responses : TStringStream ;
 PostData: TStrings;
 surl,tt :string;
 addThread :TMMThread;调用部分
surl := 'http://'+serverurl+'/game/reg/login2.php';
  addThread:=TMMThread.Create(1,1,surl,PostData, Responses);可是
thIdHTTP.Post(urlStr,ttpostdata,ttresponses);
执行异常！！
哪位高手帮忙看看啊