wysiwyg editor in template textarea

Hi,

i'm using http://drupal.org/project/htmlmail to send drupal email in HTML, it's working fine with webforms if i type html in the template area but i would like to activate the wysiwyg editor in the template area.

To do this, i'm using a simple form_alter who setup

$form['template']['template']['#wysiwyg'] = true;
$form['template']['template']['#type'] = 'text_format';

but unfortunatly, when i save the form i'm getting :
Notice: Array to string conversion in webform_email_edit_form_submit() (line 425 of /sites/all/modules/webform/includes/webform.emails.inc).

Did i forgot something in the form alter or is there a better way to achieve this ?

Thanks for answer.