Getting 400 Bad Request during POST string in spring mvc

i have a rest api that accept a String in POST and return an object,

this is the method:

@RequestMapping(method = RequestMethod.POST, value = "/aValue", headers = "Accept=application/json")
public @ResponseBody
MyObject getMyObject(@RequestBody String string) {

    MyObject response = myService.getMyObject(string);
    return response;
}

now when i call the api from another service for example, if I do POST like this it gave me always 400 Bad Request:

    List<Object> providers = new ArrayList<Object>();
    providers.add(jsonProvider);

    WebClient client = WebClient.create(baseUrl + myAPI, providers);

    client.type(MediaType.APPLICATION_JSON);
    client.accept(MediaType.APPLICATION_JSON);

    MyObject response = client.post(userId, MyObject.class);

    return response;

instead of the working solution i used which is this one:

MyObject response = client.post("\"" + userId + "\"", MyObject.class);

someone could help me ? thanks guys