array_key_exists when entity does not define label entity key

I'm getting the following error when trying to use the Geocoder field widget to encode from another field on a Message entity:

Warning: array_key_exists(): The first argument should be either a string or an integer in geocoder_field_field_widget_settings_form() (line 45 of sites/all/modules/geocoder/modules/geocoder_field/geocoder_field.module).

I traced it back to the following code:

  $title_field = $entities[$instance['entity_type']]['entity keys']['label'];
  $all_fields[$title_field] = array(
    'field_name' => $title_field,
    'type' => 'text',
  );
  $entity_fields['title']['label'] = t(drupal_ucfirst($title_field));

The Message entity doesn't define ['entity keys']['label'], it sets it to FALSE instead. I'm not sure if this is a fault with the Message module or not.