INVALID_REQUEST, 필수 파라미터 누락 (400)

        HttpHeaders httpHeaders = new HttpHeaders();
        httpHeaders.setContentType(MediaType.APPLICATION_JSON);
        httpHeaders.add("Authorization", "Basic " + api_key);

        MultiValueMap<String, String> params = new LinkedMultiValueMap<>();
        params.add("paymentKey", tossPayReadyVO.getPaymentKey());
        params.add("amount", String.valueOf(tossPayReadyVO.getAmount()));
        params.add("orderId", tossPayReadyVO.getOrderId());

        HttpEntity<?> requestData = new HttpEntity<>(params, httpHeaders);

        try {
            URI uri = URI.create(url);
            String res = restTemplate.postForObject(uri, requestData, String.class);
            System.out.println(res);
        } catch (Exception e) {
            e.printStackTrace();
        }