form preview on panel page doesn't work for list-fields

Hi,

I have a node add form embedded in a panel page. When submitting the following error occurs:

( ! ) Fatal error: Cannot unset string offsets in ... modules/field/field.default.inc on line 41

I found that the reason is that field.default.inc tries to do an unset on a string on that line:
unset($values['add_more']);
The values variable is built in the function drupal_array_get_nested_value in common.inc but it seems to create not only arrays but also a string.
Somewhere in the $ref Variable there is the following array content

"und" => 0

which maybe should be

"und" => array ("value" => 0)

I found a workaround in the field.default.inc line 41:

if ($values) unset($values['add_more']);

instead of
unset($values['add_more']);