sortable option not working

When I sort by a custom column, I get an empty list

1) I’ve bought the sortable columns option.

2) I’ve enabled hidden fields using:
add_filter(‘cpac_use_hidden_custom_fields’, ‘__return_true’);

3) I’ve added a custom field column based on a custom field that stores post IDs
– the field name begins with an underscore, and set the column to show as Post Title

This works as expected when looking at the CPT list, I see the new column, and the correct linked post titles appear.

Now, if I click the title of the column, to sort by the post titles in this custom column, I get an empty list (no posts are shown at all.)

Bug?

http://ww.wp.xz.cn/extend/plugins/codepress-admin-columns/