#states visibility applies inconsistantly

Fieldsets are missing the form-wrapper class, from form.inc:

  // The .form-wrapper class is required for #states to treat fieldsets like containers.

I was having some trouble with visibility state not applying correctly for some items (seemed to work sometimes, other times display: none; was applied to each form item rather than the fieldset itself), adding this to preprocess_fieldset appears to solved the issue:

  $vars['attributes']['class'][] = 'form-wrapper';

Just for clarify my fieldsets are in vertical tabs.