diff --git a/commons_radioactivity.module b/commons_radioactivity.module
index c076376..983540c 100644
--- a/commons_radioactivity.module
+++ b/commons_radioactivity.module
@@ -137,6 +137,9 @@ function commons_radioactivity_incident_node($node, $value) {
 * to which a node belongs.
 */
 function commons_radioactivity_incident_groups($node, $value) {
+  if (!is_array($node->og_group_ref[LANGUAGE_NONE])) {
+    return;
+  }
   foreach ($node->og_group_ref[LANGUAGE_NONE] as $field) {
     $gids[] = $field['target_id'];
   }