how to handle response get html format

Error Log cat:retrofit.RetrofitError: com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was STRING at line 1 column 1 path $

code :

void addSeatPaymnet(@Query("api.key")String key,
                        @Query("userid")String userid,
                        @Query("gender")String gender,
                        @Query("FirmaNo")String FirmaNo,
                        @Query("hatNo")String hatNo,
                        @Query("departure_city")String departure,
                        @Query("seatNo[]")String seatNo,
                        @Query("seferTakipNo")String seferTakipNo,
                        @Query("tarih")String tarih,
                        @Query("price")String price,
                        @Query("destination_city")String destination_city,
                        @Query("No_of_passenger")String No_of_passenger,
                        @Query("credit_card_holder_name")String credit_card_holder_name,
                        @Query("credit_card_number")String credit_card_number,
                        @Query("credit_card_security_code")String credit_card_security_code,
                        @Query("credit_card_expiry_month")String credit_card_expiry_month,
                        @Query("credit_card_expiry_year")String credit_card_expiry_year,
                        @Query("email")String email,
                        @Query("customer_name")String customer_name,
                        @Query("customer_suame")String customer_suame,
                        @Query("customer_mobile")String customer_mobile,
                        @Query("saat")String saat,
                        @Query("tc_number")String tc_number,
                        @Query("travelDate")String travelDate,
                        Callback<List<PaymentDetailBean> >callback);

RestClient.post().addSeatPaymnet(appConstants.api_key, "119", "2", "284", "284", "abc", "2", "687036", "2016-02-26", "75", "bcd",
                "1", "abdusl", "4222 3160 8933 0000", "242", "11", "2018", "patel@gmail.com",
                "abc", "raul", "312155555", "MTkwMC0wMS0wMV==", "1900-01-01T05:00:00-02:00", "07 March 2016 - 10:00", new Callback<List<PaymentDetailBean>>() {
                    @Override
                    public void success(List<PaymentDetailBean> paymentDetailBeans, Response response) {
                        Log.d("success", "1"+response);
                    }

                    @Override
                    public void failure(RetrofitError error) {
                        Log.d("success", "2" + error);
                    }
                });