Flutter FormatException: Unexpected character (at character 2)

  void _requestLoginAction() async {
    Response response;
    final dio = Dio();
    dio.options.baseUrl = 'https://ams.e-next.cn/api';
    dio.options.connectTimeout = Duration(seconds: 5);
    dio.options.receiveTimeout = Duration(seconds: 3);
    dio.options.responseType = ResponseType.plain;
    response = await dio.post('/app/login',
        data: {'username': _email, 'password': _password});
    debugPrint(response.data.toString());
    var map = json.decode(response.data.toString());
    var aModel = LoginModelEntity.fromJson(map);
    if (aModel.code == 200 ){
      debugPrint(aModel.data?.token);
    }
   
  }