POST request with Retrofit giving 500

I have the following Body sent via postman return just fine (as raw JSON)

{"payload":{"email":"","username":"","password":""}}

However when i try to send it via Retrofit as follows, it doesn't work (gives 500):

public Observable<JsonObject> signup(String email, String userName, String password) {
    JsonObject data = new JsonObject();
    JsonObject payload = new JsonObject();

    data.addProperty("email", email);
    data.addProperty("username", userName);
    data.addProperty("password", password);
    payload.add("payload", data);

    return mShoutApiService.signup(payload);
}

Service call and adapter are as follows

service:

@POST("/signup")
Observable<JsonObject> signup(@Body JsonObject payload);

adapter:

private RestAdapter createRestAdapter(String hostUrl, GsonConverter gsonConverter, OkClient okClient) {
    return new RestAdapter.Builder()
            .setClient(okClient)
            .setConverter(gsonConverter)
            .setEndpoint(hostUrl)
            .setLogLevel(LogLevel.FULL)
            .setLog(new AndroidLog("SHOUT_LOG"))
            .build();
}

private GsonConverter getGsonConverter(Gson gson) {
    return new GsonConverter(gson);
}

private OkClient getOkClient() {
    OkHttpClient okHttpClient = new OkHttpClient();
    okHttpClient.setReadTimeout(READ_TIMEOUT, TimeUnit.SECONDS);
    okHttpClient.setConnectTimeout(CONNECT_TIMEOUT, TimeUnit.SECONDS);

    return new OkClient(okHttpClient);
}

Logs show as follows

03-23 05:04:00.380 2442-3468/shout.surge.com.shout D/SHOUT_LOG: --->       HTTP POST http://shout-api.herokuapp.com/signup
03-23 05:04:00.380 2442-3468/shout.surge.com.shout D/SHOUT_LOG: Content-Type: application/json; charset=UTF-8
03-23 05:04:00.380 2442-3468/shout.surge.com.shout D/SHOUT_LOG: Content-Length: 52
03-23 05:04:00.384 2442-3468/shout.surge.com.shout D/SHOUT_LOG: {"payload":{"email":"","username":"","password":""}}
03-23 05:04:00.384 2442-3468/shout.surge.com.shout D/SHOUT_LOG: ---> END HTTP (52-byte body)
03-23 05:04:09.180 2442-3468/shout.surge.com.shout D/SHOUT_LOG: <--- HTTP  500 http://shout-api.herokuapp.com/signup (8796ms)

It appears they are the exact same call but for some reason it fails in my mobile app. any insight as to what I'm missing would be great!