diff --git a/core/modules/history/lib/Drupal/history/HistoryManager.php b/core/modules/history/lib/Drupal/history/HistoryManager.php
index 7510afd..64887d9 100644
--- a/core/modules/history/lib/Drupal/history/HistoryManager.php
+++ b/core/modules/history/lib/Drupal/history/HistoryManager.php
@@ -76,6 +76,9 @@ public function read($entity_type, $entity_ids) {
     foreach ($result as $row) {
       $entities_to_read[$row->entity_id] = $row->timestamp;
     }
+    if (!isset($this->history[$entity_type])) {
+      $this->history[$entity_type] = array();
+    }
     $this->history[$entity_type] += $entities_to_read;
 
     return $return + $entities_to_read;