diff --git a/src/Symfony/Bundle/SecurityBundle/composer.json b/src/Symfony/Bundle/SecurityBundle/composer.json
index a222c68ac1..0943db9af7 100644
--- a/src/Symfony/Bundle/SecurityBundle/composer.json
+++ b/src/Symfony/Bundle/SecurityBundle/composer.json
@@ -44,7 +44,7 @@
         "twig/twig": "~1.34|~2.4"
     },
     "conflict": {
-        "symfony/security": "4.1.0-beta1,4.1.0-beta2",
+        "symfony/security": "4.1.0-beta1|4.1.0-beta2",
         "symfony/var-dumper": "<3.4",
         "symfony/event-dispatcher": "<3.4",
         "symfony/framework-bundle": "<4.1",
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/CheckExceptionOnInvalidReferenceBehaviorPass.php b/src/Symfony/Component/DependencyInjection/Compiler/CheckExceptionOnInvalidReferenceBehaviorPass.php
index 1bae24c10b..b13f2bdc5f 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/CheckExceptionOnInvalidReferenceBehaviorPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/CheckExceptionOnInvalidReferenceBehaviorPass.php
@@ -11,7 +11,6 @@
 
 namespace Symfony\Component\DependencyInjection\Compiler;
 
-use Symfony\Component\DependencyInjection\Exception\InvalidArgumentException;
 use Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\DependencyInjection\Reference;
@@ -31,9 +30,6 @@ class CheckExceptionOnInvalidReferenceBehaviorPass extends AbstractRecursivePass
         if (ContainerInterface::EXCEPTION_ON_INVALID_REFERENCE >= $value->getInvalidBehavior() && !$this->container->has($id = (string) $value)) {
             throw new ServiceNotFoundException($id, $this->currentId);
         }
-        if (ContainerInterface::IGNORE_ON_UNINITIALIZED_REFERENCE === $value->getInvalidBehavior() && $this->container->has($id = (string) $value) && !$this->container->findDefinition($id)->isShared()) {
-            throw new InvalidArgumentException(sprintf('Invalid ignore-on-uninitialized reference found in service "%s": target service "%s" is not shared.', $this->currentId, $id));
-        }
 
         return $value;
     }
diff --git a/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php b/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
index 8491f806b4..cfe9fdfcfb 100644
--- a/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
+++ b/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
@@ -1660,6 +1660,9 @@ EOF;
         if ($this->container->hasDefinition($id) && ($definition = $this->container->getDefinition($id)) && !$definition->isSynthetic()) {
             if (null !== $reference && ContainerInterface::IGNORE_ON_UNINITIALIZED_REFERENCE === $reference->getInvalidBehavior()) {
                 $code = 'null';
+                if (!$definition->isShared()) {
+                    return $code;
+                }
             } elseif ($this->isTrivialInstance($definition)) {
                 $code = substr($this->addNewInstance($definition, '', '', $id), 8, -2);
                 if ($definition->isShared()) {
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Compiler/CheckExceptionOnInvalidReferenceBehaviorPassTest.php b/src/Symfony/Component/DependencyInjection/Tests/Compiler/CheckExceptionOnInvalidReferenceBehaviorPassTest.php
index ac002d834d..a3fbfcf101 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Compiler/CheckExceptionOnInvalidReferenceBehaviorPassTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Compiler/CheckExceptionOnInvalidReferenceBehaviorPassTest.php
@@ -68,27 +68,6 @@ class CheckExceptionOnInvalidReferenceBehaviorPassTest extends TestCase
         $this->process($container);
     }
 
-    /**
-     * @expectedException \Symfony\Component\DependencyInjection\Exception\InvalidArgumentException
-     * @expectedExceptionMessage Invalid ignore-on-uninitialized reference found in service
-     */
-    public function testProcessThrowsExceptionOnNonSharedUninitializedReference()
-    {
-        $container = new ContainerBuilder();
-
-        $container
-            ->register('a', 'stdClass')
-            ->addArgument(new Reference('b', $container::IGNORE_ON_UNINITIALIZED_REFERENCE))
-        ;
-
-        $container
-            ->register('b', 'stdClass')
-            ->setShared(false)
-        ;
-
-        $this->process($container);
-    }
-
     public function testProcessDefinitionWithBindings()
     {
         $container = new ContainerBuilder();
diff --git a/src/Symfony/Component/HttpFoundation/File/MimeType/MimeTypeGuesser.php b/src/Symfony/Component/HttpFoundation/File/MimeType/MimeTypeGuesser.php
index e3ef45ef67..d78c760682 100644
--- a/src/Symfony/Component/HttpFoundation/File/MimeType/MimeTypeGuesser.php
+++ b/src/Symfony/Component/HttpFoundation/File/MimeType/MimeTypeGuesser.php
@@ -80,13 +80,8 @@ class MimeTypeGuesser implements MimeTypeGuesserInterface
      */
     private function __construct()
     {
-        if (FileBinaryMimeTypeGuesser::isSupported()) {
-            $this->register(new FileBinaryMimeTypeGuesser());
-        }
-
-        if (FileinfoMimeTypeGuesser::isSupported()) {
-            $this->register(new FileinfoMimeTypeGuesser());
-        }
+        $this->register(new FileBinaryMimeTypeGuesser());
+        $this->register(new FileinfoMimeTypeGuesser());
     }
 
     /**
@@ -125,18 +120,14 @@ class MimeTypeGuesser implements MimeTypeGuesserInterface
             throw new AccessDeniedException($path);
         }
 
-        if (!$this->guessers) {
-            $msg = 'Unable to guess the mime type as no guessers are available';
-            if (!FileinfoMimeTypeGuesser::isSupported()) {
-                $msg .= ' (Did you enable the php_fileinfo extension?)';
-            }
-            throw new \LogicException($msg);
-        }
-
         foreach ($this->guessers as $guesser) {
             if (null !== $mimeType = $guesser->guess($path)) {
                 return $mimeType;
             }
         }
+
+        if (2 === \count($this->guessers) && !FileBinaryMimeTypeGuesser::isSupported() && !FileinfoMimeTypeGuesser::isSupported()) {
+            throw new \LogicException('Unable to guess the mime type as no guessers are available (Did you enable the php_fileinfo extension?)');
+        }
     }
 }