--- sites/all/modules/gmap/gmap.views.inc.orig	Sun Aug 21 14:11:50 2011
+++ sites/all/modules/gmap/gmap.views.inc	Sun Aug 21 14:12:58 2011
@@ -50,6 +50,9 @@
   foreach ($renders as $i => $row) {
     $markers[$i] = $options['fallback_values'];
     foreach ($options['field_purposes'] as $field => $purpose) {
+      if (!isset($markers[$i][$purpose])) {
+        $markers[$i][$purpose] = '';
+      }
       $markers[$i][$purpose] .= $row[$field];
     }
   }