RX如何结合使用application/json？

界面

public interface IApiRequest {

    @POST("/v1/register")
    Observable<ResponseBody> makeRegister(@Body ApiBody.MakeRegister register);
}

要求

public class ApiRequest {

    private static IApiRequest init(){
        return new Retrofit.Builder()
                .baseUrl("https://slife.auth.andviro.pw")
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .build().create(IApiRequest.class);
    }

    public static Observable<ResponseBody> makeRegister(String email, String password){
        return init().makeRegister(new ApiBody.MakeRegister(email,password));
    }
}

挑战

ApiRequest.makeRegister("chhdjd@mail.ru","sadasd22")
                .doOnSubscribe(disposable -> {})
                .doFinally(() -> {})
                .subscribe(responseBody -> {
                    Log.e("Test", "main= " + responseBody.string());
                },Throwable::printStackTrace).dispose();

启动时出现错误

Exception in thread "main" java.lang.IllegalArgumentException: Unable to create @Body converter for class com.eranewgames.slife.network.ApiBody$MakeRegister (parameter #1)
for method IApiRequest.makeRegister

检查 PostMan 中的代码后，一切正常。我在代码中做错了什么？以前，我只使用表单/数据类型。这是应用程序/json