diff --git a/src/DelegatingContainer.php b/src/DelegatingContainer.php
index 0b36cd0..7d264a8 100644
--- a/src/DelegatingContainer.php
+++ b/src/DelegatingContainer.php
@@ -34,7 +34,7 @@ class DelegatingContainer implements ContainerInterface
 
     /**
      */
-    public function __construct(ServiceProviderInterface $provider, PsrContainerInterface $parent = null)
+    public function __construct(ServiceProviderInterface $provider, ?PsrContainerInterface $parent = null)
     {
         $this->provider = $provider;
         $this->parent = $parent;
diff --git a/src/Exception/NotFoundException.php b/src/Exception/NotFoundException.php
index 8f14cd8..0f448fb 100644
--- a/src/Exception/NotFoundException.php
+++ b/src/Exception/NotFoundException.php
@@ -10,14 +10,14 @@
 class NotFoundException extends ContainerException implements NotFoundExceptionInterface
 {
     /**
-     * @param string         $message  The error message.
-     * @param int            $code     The error code.
-     * @param Throwable|null $previous The inner error, if any.
+     * @param string     $message  The error message.
+     * @param int        $code     The error code.
+     * @param ?Throwable $previous The inner error, if any.
      */
     public function __construct(
         string $message = '',
         int $code = 0,
-        Throwable $previous = null
+        ?Throwable $previous = null
     ) {
         parent::__construct($message, $code, $previous);
     }
diff --git a/src/ProxyContainer.php b/src/ProxyContainer.php
index b65e5b7..45ab7ef 100644
--- a/src/ProxyContainer.php
+++ b/src/ProxyContainer.php
@@ -24,10 +24,10 @@ class ProxyContainer implements BaseContainerInterface
     protected $innerContainer;
 
     /**
-     * @param BaseContainerInterface|null $innerContainer The inner container, if any.
-     *                                                    May also be set later with {@see setInnerContainer()}.
+     * @param ?BaseContainerInterface $innerContainer The inner container, if any.
+     *                                                May also be set later with {@see setInnerContainer()}.
      */
-    public function __construct(BaseContainerInterface $innerContainer = null)
+    public function __construct(?BaseContainerInterface $innerContainer = null)
     {
         $this->innerContainer = $innerContainer;
     }