only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Flood/MemoryBackend.php
+++ b/core/lib/Drupal/Core/Flood/MemoryBackend.php
@@ -58,6 +58,9 @@ public function clear($name, $identifier = NULL) {
    * {@inheritdoc}
    */
   public function isAllowed($name, $threshold, $window = 3600, $identifier = NULL) {
+    if (!isset($this->events[$name])) {
+      return $threshold > 0;
+    }
     if (!isset($identifier)) {
       $identifier = $this->requestStack->getCurrentRequest()->getClientIp();
     }