MockResponse setBody not working (OKHttp)

Good day. I'm writing simple test with API calls where Retrofit takes place.

The test looks like this

 mockWebServer.enqueue(new MockResponse()
                .setResponseCode(200)
                .setBody("{\"initial\":\"合理咯\"," +
                        "  \"translation\":\"sometranlsation\"}"));
        PinyTranslationRequest pinyTranslationRequest = new PinyTranslationRequest();

        ArrayList<String> inputString = new ArrayList<>();
        inputString.add("合理咯");

        pinyTranslationRequest.locale = CHINESSE_LANGUAGE_COUNTRY_CODE;
        pinyTranslationRequest.inputStrings = inputString;

        Call<PinyinTranslationResponse> responseCall = backendServerAPI.getPinyinTranslation(pinyTranslationRequest);

        Response<PinyinTranslationResponse> pinyinTranslationResponseResponse = responseCall.execute();

        assertEquals("he li ge", pinyinTranslationResponseResponse.body().translation);

Here the assertion fails becausetranslation variable is null so I've checked the full response which looks as follows:

Response{protocol=h2, code=200, message=, url=https://xxxx.xxx}

As you can see message is empty but whereas it shall not as I've set the body for the MockResponse.

Any clues what am I doing wrong?