Unsetting $element['#post'] causes notice

When I render a page with a hierarchical select on it I get the following notice:

Notice: Undefined index: #post in form_builder()

I found that in hierarchical_select_process() $element['#post'] is being unset which results in the notice. Changing the code to set $element['post'] to an empty array resolves the issue and achieves the same result as unsetting it.