diff --git a/core/lib/Drupal/Core/Authentication/AuthenticationManager.php b/core/lib/Drupal/Core/Authentication/AuthenticationManager.php
index 17498ba..d95873d 100644
--- a/core/lib/Drupal/Core/Authentication/AuthenticationManager.php
+++ b/core/lib/Drupal/Core/Authentication/AuthenticationManager.php
@@ -52,7 +52,12 @@ public function applies(Request $request) {
   public function authenticate(Request $request) {
     $provider_id = $this->getProvider($request);
     $provider = $this->authCollector->getProvider($provider_id);
-    return $provider->authenticate($request);
+
+    if ($provider) {
+      return $provider->authenticate($request);
+    }
+
+    return NULL;
   }
 
   /**
@@ -82,7 +87,8 @@ public function appliesToRoutedRequest(Request $request, $authenticated) {
   public function challengeException(Request $request, \Exception $previous) {
     $provider_id = $this->getChallenger($request);
 
-    if ($provider_id && ($provider = $this->authCollector->getProvider($provider_id))) {
+    if ($provider_id) {
+      $provider = $this->authCollector->getProvider($provider_id);
       return $provider->challengeException($request, $previous);
     }
   }
diff --git a/core/tests/Drupal/Tests/Core/Authentication/AuthenticationCollectorTest.php b/core/tests/Drupal/Tests/Core/Authentication/AuthenticationCollectorTest.php
index bfc8d51..609f4f3 100644
--- a/core/tests/Drupal/Tests/Core/Authentication/AuthenticationCollectorTest.php
+++ b/core/tests/Drupal/Tests/Core/Authentication/AuthenticationCollectorTest.php
@@ -19,7 +19,7 @@
 class AuthenticationCollectorTest extends UnitTestCase {
 
   /**
-   * Add test providers to the AuthenticationCollector and test various methods.
+   * Tests adding, getting, and order of priorities.
    *
    * @covers ::addProvider
    * @covers ::getSortedProviders
@@ -67,7 +67,7 @@ class TestAuthenticationProvider implements AuthenticationProviderInterface {
   /**
    * The provider id.
    *
-   * @var $provider_id
+   * @var string
    */
   public $providerId;