Undefined index: group in _wysiwyg_pre_render_styles() with AdvAgg module

We use AdvAgg for managing our CSS delivery and this seems to cause problems to WYSIWYG, namely in calls to the special URL /wyiwyg_theme/[theme name].

The page returns OK or Failed depending on whether or not the theme name is valid, but given the way AdvAgg creates its aggregated files, the code trips up because $elements['#groups'][$child]['group'] is not set. There is no group element in $child.

I guess adding an isset() would prevent the warning, but does the fact of using AdvAgg break what this page is supposed to do in any case? I've not had any end-users report broken functionality or funky page layout anywhere.

Regards,

- Paul