Android Retrofit response.body() is always null

Question

I'm having a problem with Retrofit.

When I make a call to my REST API, every time the response.body() is not successfull and null

I tried to make the same call with Postman and the body contains the correct answer.

This is my api Interface:

public interface Api{
    //User Login
    @FormUrlEncoded
    @POST("standard")
    Call<ResponseBody> standard(
            @Field("latitude") String latitude,
            @Field("longitude") String longitude
    );
}

My request:

public void updateListView(String latitude, String longitude) {
        Call<ResponseBody> call = RetrofitClient
                .getInstance()
                .getApi()
                .standard(
                        latitude,
                        longitude
                );

        call.enqueue(new Callback<ResponseBody>() {
            @Override
            public void onResponse(Call<ResponseBody> call, Response<ResponseBody> response) {
                if(response.isSuccessful()) {
                    //the response-body is already parseable to your ResponseBody object
                    ResponseBody responseBody = (ResponseBody) response.body();
                    //you can do whatever with the response body now...
                    String responseBodyString= null;
                    try {
                        responseBodyString = responseBody.string();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                    Log.d("Response body", responseBodyString);
                }
                else  {
                    Log.d("Response errorBody", String.valueOf(response.errorBody()));
                }
            }

            @Override
            public void onFailure(Call<ResponseBody> call, Throwable t) {
                Log.e("onFalure", t.getMessage());
                Toast.makeText(TestRequest.this, t.getMessage(), Toast.LENGTH_LONG).show();
            }
        });
    }

Error Code:

D/Response errorBody: okhttp3.ResponseBody$1@7ea6319

My successfull postman request: