diff --git a/core/lib/Drupal/Core/Flood/MemoryBackend.php b/core/lib/Drupal/Core/Flood/MemoryBackend.php
index ddb42bc..b80e7f2 100644
--- a/core/lib/Drupal/Core/Flood/MemoryBackend.php
+++ b/core/lib/Drupal/Core/Flood/MemoryBackend.php
@@ -61,6 +61,9 @@ public function isAllowed($name, $threshold, $window = 3600, $identifier = NULL)
     if (!isset($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;
diff --git a/core/modules/system/src/Tests/System/FloodTest.php b/core/modules/system/src/Tests/System/FloodTest.php
index a3a3368..bba3032 100644
--- a/core/modules/system/src/Tests/System/FloodTest.php
+++ b/core/modules/system/src/Tests/System/FloodTest.php
@@ -34,8 +34,9 @@ public function testCleanUp() {
     $window_expired = -1;
     $name = 'flood_test_cleanup';
 
-    // Register expired event.
     $flood = \Drupal::flood();
+    $this->assertTrue($flood->isAllowed($name, $threshold));
+    // Register expired event.
     $flood->register($name, $window_expired);
     // Verify event is not allowed.
     $this->assertFalse($flood->isAllowed($name, $threshold));
@@ -62,6 +63,7 @@ public function testMemoryBackend() {
 
     $request_stack = \Drupal::service('request_stack');
     $flood = new MemoryBackend($request_stack);
+    $this->assertTrue($flood->isAllowed($name, $threshold));
     // Register expired event.
     $flood->register($name, $window_expired);
     // Verify event is not allowed.
@@ -90,6 +92,7 @@ public function testDatabaseBackend() {
     $connection = \Drupal::service('database');
     $request_stack = \Drupal::service('request_stack');
     $flood = new DatabaseBackend($connection, $request_stack);
+    $this->assertTrue($flood->isAllowed($name, $threshold));
     // Register expired event.
     $flood->register($name, $window_expired);
     // Verify event is not allowed.