Getting UnsupportedMediaTypeException while calling an API endpoint to fetch the Exception response

I have to call a third-party endpoint in my service and need to fetch the errors directly from the endpoint while an error has occurred. I am using WebClient to call the endpoint and below is the snippet.

webClient.post()
        .uri(uri)
        .header(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE)
        .header(HttpHeaders.ACCEPT, MediaType.APPLICATION_JSON_VALUE)
        .bodyValue(request)
        .retrieve()
        .onStatus(BAD_REQUEST::equals, clientResponse -> clientResponse
                        .bodyToMono(ApiException.class)
                        .map(ApiException::getErrors)
                        .map(e -> e.get(0).getDetail())
                        .map(TestException::new));

Getting below error while mapping the exception response: [Request processing failed; nested exception is org.springframework.web.reactive.function.UnsupportedMediaTypeException: Content type 'application/json' not supported for bodyType=com.squareup.square.exceptions.ApiException] with root cause.

Any help would be appreciated.Thanks!