jquery - select all checkboxes under tbody not working

I'm not sure why this jquery is not working. What I want is pretty straightforward: selecting all check boxes under the region. Could someone help tell why it's not working? Thanks a lot!

$(function(){
  $('.select_all').change(function() {
    var checkthis = $(this);
    var checkboxes = $(this).next('tbody').find('.region_ct');
    if(checkthis.is(':checked')) {
      checkboxes.attr('checked', true);
    } else {
      checkboxes.attr('checked', false);  }
  });
});

<script src="http://ift.tt/1oMJErh"></script>
<table class="table">
  <tr>
    <th colspan=2>Americas</th>
  </tr>
  <tr>
    <td colspan=2><input type="checkbox" name="region_am_all" class="select_all" value="X" /> Select All</td>
  </tr>
  <tbody>
    <tr>
      <td><input type="checkbox" class="region_ct" value="X"  /> Argentina</td>
      <td class="center"></td>
    </tr>
    <tr>
      <td><input type="checkbox" class="region_ct" value="X"  /> Barbados</td>
      <td class="center"></td>
    </tr>
  </tbody>
  <tr>
    <th colspan=2>Asia</th>
  </tr>
  ...
</table>