See: http://drupal.org/node/1103790

diff -Nurp ../date.orig//date_views/includes/date_views_fields.inc ./date_views/includes/date_views_fields.inc
--- ../date.orig//date_views/includes/date_views_fields.inc	2011-03-28 08:28:27.000000000 -0500
+++ ./date_views/includes/date_views_fields.inc	2011-03-28 10:13:19.000000000 -0500
@@ -58,7 +58,7 @@ function _date_views_fields($base = 'nod
 
     // For Field module fields, get the date type.
     $custom = array();
-    if ($handler->definition['group'] == t('Fields')) {
+    if (isset($handler->definition['group']) && $handler->definition['group'] == t('Fields')) {
       $field = field_info_field($handler->definition['field_name']);
       switch ($field['type']) {
        case 'date':