Spring MVC post List of ImmutablePair with Ajax

I have a service method that takes List<ImmutablePair<Long, Long>> but can't get the controller to accept (bind? my jargon isn't great) the data. Simplified version of what I currently have:

Object:

public class Dto {
    private List<ImmutablePair<Long, Long>> foo;

    ...getters and setters

}

Controller:

@RequestMapping(value = "list", method = RequestMethod.POST, headers = "Accept=application/json")
@ResponseBody
public String postInventoryList(@RequestBody Dto dto) {
    ...
}

JS:

$http.post(listUrl, JSON.stringify({foo: [{left:1, right:1}]})).then(function (result) {
    console.log(result);
});

I can get it to work if I pass the pairs differently, but I'd really like to just use ImmutablePair to keep things clean and avoid having to process the data before calling that method. I'm guessing I need to tell Spring how to deal with this object type? How would I do this? Thanks!