Why this always return "undefined"

Why this always return "undefined"

Hi every, I have this two HTML select (consigna_id depends on manufacturer_id selection):
  1. <select id="manufacturer_id" name="manufacturer_id">
  2.  <option selected="selected" value="0"> --- Ninguno --- </option>
  3.  <option value="6">Júpiter Ediciones 9. C.A.</option>
  4.  <option value="2">Proveedor 1</option>
  5.  <option value="3">Proveedor 2</option>
  6.  <option value="5">Proveedor 3</option>
  7.  <option value="4">Proveedor 4</option>
  8. </select>
  9. <select disabled="disabled" id="consigna_id" name="consigna_id">
  10. </select>
And then have this jQuery code to get data dinamically via AJAX:
  1. // Search consigna based on manufacturer
  2. $("#manufacturer_id").change(function(){
  3.   $.ajax({
  4.     url: 'index.php?route=catalog/product/getConsignaManufacturer& token=11a3c350e29c157da32ea947b3aa9b29&manufacturer_id=' + $('#manufacturer_id').val(),
  5.     type: 'GET',
  6.     dataType: 'json',
  7.     success: function(dato){
  8.     alert(dato.consigna_id);
  9.     $("#consigna_id").empty().removeAttr("disabled").append(dato.consigna_code);
  10.   }
  11. });
  12. });
The function return data as follow:
[{"consigna_id":"4","manufacturer_id":"2","consigna_code":"f3150877","date":"2011-03-03","date_added":"0000-00-00 00:00:00","date_modified":"0000-00-00 00:00:00"}]
But the message "undefined" appears any time I try to access some JSON property, for example dato.consigna_id wich is fine because I use the same sintaxis in others parts and works fine. Why this fail?
Cheers and thanks in advance