UnicodeEncodeError in send_documents form (EmailContactsAndUsersVocabularyFactory)

dossier-xy/send_documents/++widget++intern_receiver/@@autocomplete-search
Traceback (innermost last):
 Module ZPublisher.Publish, line 126, in publish
 Module ZPublisher.mapply, line 77, in mapply
 Module ZPublisher.Publish, line 46, in call_object
 Module plone.formwidget.autocomplete.widget, line 66, in __call__
 Module collective.elephantvocabulary.base, line 28, in _wrap_vocab
 Module opengever.ogds.base.vocabulary, line 30, in search
 Module zope.schema.vocabulary, line 111, in createTerm
 Module zope.schema.vocabulary, line 41, in __init__
UnicodeEncodeError: 'ascii' codec can't encode character u'\xe4' in position 21: ordinal not in range(128)