不能json_encode表单数据

I got a ajax request to a method, where I send some data to.

My ajax call is as follows:

$.ajax({
        url: btn.data('url'),
        type: 'POST',
        data: {_method: 'delete', _token: token, items: JSON.stringify(_items)},
        success: function () {
            //
        }
    })

Where token is read from a meta tag and _items is an array with ID's.

When I inspect this ajax call, the form data contains:

_method:delete
_token:15KeBPmbt7WVgm8KNCWG4idqCw3FpGykzREPn8fu
items: ["4","11","15"]

But when I dd(json_decode(Input::get('items') it returns:

0: 3
1: 0
2: 4
3: 1
4: 1
5: 11
6: 2
7: 2
8: 15
9: 2

Edit: when I remove the JSON.stringify the formdata contains:

items[]:1
items[]:2

and returns a array of:

0: 2
1: 0
2: 1
3: 1
4: 1
5: 2
6: 1

Here is my route catch:

Route::delete('user/destroy/multiple', function() {
    dd(Input::get('items'));
});

I really don't know what going wrong...