diff -u b/core/lib/Drupal/Core/Flood/MemoryBackend.php b/core/lib/Drupal/Core/Flood/MemoryBackend.php
--- b/core/lib/Drupal/Core/Flood/MemoryBackend.php
+++ b/core/lib/Drupal/Core/Flood/MemoryBackend.php
@@ -58,12 +58,12 @@
    * {@inheritdoc}
    */
   public function isAllowed($name, $threshold, $window = 3600, $identifier = NULL) {
-    if (!isset($this->events[$name])) {
-      return $threshold > 0;
-    }
     if (!isset($identifier)) {
       $identifier = $this->requestStack->getCurrentRequest()->getClientIp();
     }
+    if (!isset($this->events[$name][$identifier])) {
+      return TRUE;
+    }
     $limit = microtime(TRUE) - $window;
     $number = count(array_filter($this->events[$name][$identifier], function ($timestamp) use ($limit) {
       return $timestamp > $limit;