diff --git a/src/Symfony/Component/Security/Http/Authenticator/RemoteUserAuthenticator.php b/src/Symfony/Component/Security/Http/Authenticator/RemoteUserAuthenticator.php
index 140b6c271efb..96890e1aacf8 100644
--- a/src/Symfony/Component/Security/Http/Authenticator/RemoteUserAuthenticator.php
+++ b/src/Symfony/Component/Security/Http/Authenticator/RemoteUserAuthenticator.php
@@ -45,6 +45,6 @@ protected function extractUsername(Request $request): ?string
             throw new BadCredentialsException(sprintf('User key was not found: "%s".', $this->userKey));
         }
 
-        return $request->server->get($this->userKey);
+        return $request->server->get($this->userKey) ?: null;
     }
 }
diff --git a/src/Symfony/Component/Security/Http/Tests/Authenticator/RemoteUserAuthenticatorTest.php b/src/Symfony/Component/Security/Http/Tests/Authenticator/RemoteUserAuthenticatorTest.php
index 5119f8ce09e7..f16ea197de5d 100644
--- a/src/Symfony/Component/Security/Http/Tests/Authenticator/RemoteUserAuthenticatorTest.php
+++ b/src/Symfony/Component/Security/Http/Tests/Authenticator/RemoteUserAuthenticatorTest.php
@@ -36,6 +36,7 @@ public function testSupportNoUser()
         $authenticator = new RemoteUserAuthenticator(new InMemoryUserProvider(), new TokenStorage(), 'main');
 
         $this->assertFalse($authenticator->supports($this->createRequest([])));
+        $this->assertFalse($authenticator->supports($this->createRequest(['REMOTE_USER' => ''])));
     }
 
     public function testSupportTokenStorageWithToken()
@@ -63,7 +64,7 @@ public function testAuthenticate(InMemoryUserProvider $userProvider, RemoteUserA
         $passport = $authenticator->authenticate($request);
         $this->assertTrue($user->isEqualTo($passport->getUser()));
     }
-
+    
     public static function provideAuthenticators()
     {
         $userProvider = new InMemoryUserProvider();