Sending form using Ajax - Spring MVC

I have problems with sending my form using Ajax.

Here is form:

<form method="POST" id="add-card-form" action="${pageContext.request.contextPath}/card/add" class="form-horizontal">
 <select name="type" class="form-control">
    <c:forEach items="${cardTypes}" var="cardType">
       <option value="${cardType.id}">${cardType.name}</option>
    </c:forEach>
 </select>
 <select name="category" class="form-control">
    <c:forEach items="${cardCategories}" var="cardCategory">
       <option value="${cardCategory.id}">${cardCategory.name}</option>
    </c:forEach>
 </select>
<textarea type="text" name="description" class="form-control" rows="6"></textarea>
 <input type="submit" id="add-card-submit" value="Add card" class="btn btn-primary"/>

Here is Ajax function:

$(document).on('submit', '#add-card-form', function(e) {
    var frm = $('#add-card-form');
    e.preventDefault();

    var Form = this;
    var data = {};

    $.each(this, function(i, v){
        var input = $(v);
        data[input.attr("name")] = input.val();
        delete data["undefined"];
    });

    //temporary solution
    data["type"] = parseInt(data["type"]);
    data["category"] = parseInt(data["category"]);

    console.log(data);
    if(frm.valid()) {
        $.ajax({
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            type: frm.attr('method'),
            url: frm.attr('action'),
            data: JSON.stringify(data),
            success:  reloadBoard,
            error: function (callback) {
                console.log(callback);
            }
        });

        refreshForm(frm);
    }
});

And here is a controller action:

@RequestMapping(value="/add", method = RequestMethod.POST)
public @ResponseBody Card addCard(@RequestBody Integer type,
                                  @RequestBody Integer category,
                                  @RequestBody String description) {

    Card card = new Card();

    card.setType(cardTypeService.findById(type));
    card.setCategory(cardCategoryService.findById(category));
    card.setDescription(description);
    card.setOwner(1);

    cardService.saveCard(card);

    System.out.println("Card with id " + card.getId() + " added!");

    return card;
}

Variable data values:

Object {type: 1, category: 1, description: "New Card"}

When I try to send this form I always get error 400: http://localhost:8080/card/add 400 (Bad Request)

Can you tell me what is wrong with this code? I've ridden few posts, articles about sending data using Spring MVC + Ajax but no one helped.