strange problem with dajaxice and javascript

hi all

i have a javascript callback function that return a data json string from a dajaxice view

this is the callback function

function message_callback(data){
    alert(data.id);
    var id_book = data.id;
    var uploader = $('#uploader').pluploadQueue({
            url : '/admin/ecommerce/books/images/upload/'+id_book.toString()+'/'
    });
    ...........

the aleet return the correct id number
but the url is : /admin/ecommerce/books/images/upload/undefined

have someting wrong?
sorry if you think this is a stupid question but i have lost a lot of time googleing this problem without any result

have a nice day

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