Cannot encrypt select list fields

There is no possibility to use encryption settings in list fields. The 'Encrypt this field' checkbox does not appears.

I think the problem is in the module file, line 68:

  // Check that there are encryptable columns on this field as Indexes can not
  // be encrypted.
  if (array_keys($field_info['columns']) == array_keys($field_info['indexes'])) {
    return;
  }

For a list field, the 'indexes' and 'columns' contains the same 'value' field.

I would like to understand why indexes cannot be encrypted? Is there any workaround to this problem?

Thanks