diff --git a/core/modules/tracker/tracker.pages.inc b/core/modules/tracker/tracker.pages.inc
index eeef432..19f93ff 100644
--- a/core/modules/tracker/tracker.pages.inc
+++ b/core/modules/tracker/tracker.pages.inc
@@ -126,6 +126,9 @@ function tracker_page($account = NULL, $set_title = FALSE) {
         // modified or a comment was posted, so we use only 'last_activity'.
         $last_activity_mapping = $mapping->getPreparedFieldMapping('last_activity');
         $last_activity_attributes = rdf_rdfa_attributes($last_activity_mapping, $node->last_activity);
+        $last_activity_attributes += array(
+          'property' => array(),
+        );
         if ($node->comment_count == 0) {
           $changed_mapping = $mapping->getPreparedFieldMapping('changed');
           $changed_attributes = rdf_rdfa_attributes($changed_mapping, $node->last_activity);