diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 39d1e16..6c23c75 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -33,6 +33,8 @@ jobs:
                 include:
                     - php: '8.0'
                       coverage: false
+                    - php: '8.4'
+                      coverage: false
                     - php: '7.2'
                       coverage: false
                       composer-flags: '--prefer-lowest'
@@ -53,7 +55,7 @@ jobs:
 
             - name: "Use PHPUnit 9.3+ on PHP 8"
               run: composer require --no-update --dev phpunit/phpunit:^9.3
-              if: "matrix.php == '8.0'"
+              if: "matrix.php == '8.0' || matrix.php == '8.4'"
 
             - run: composer update --no-progress ${{ matrix.composer-flags }}
 
diff --git a/src/Argument/LiteralArgument.php b/src/Argument/LiteralArgument.php
index c121d35..55adede 100644
--- a/src/Argument/LiteralArgument.php
+++ b/src/Argument/LiteralArgument.php
@@ -24,7 +24,7 @@ class LiteralArgument implements LiteralArgumentInterface
      */
     protected $value;
 
-    public function __construct($value, string $type = null)
+    public function __construct($value, ?string $type = null)
     {
         if (
             null === $type
diff --git a/src/Container.php b/src/Container.php
index c3aa35d..195dba7 100644
--- a/src/Container.php
+++ b/src/Container.php
@@ -40,9 +40,9 @@ class Container implements DefinitionContainerInterface
     protected $delegates = [];
 
     public function __construct(
-        DefinitionAggregateInterface $definitions = null,
-        ServiceProviderAggregateInterface $providers = null,
-        InflectorAggregateInterface $inflectors = null
+        ?DefinitionAggregateInterface $definitions = null,
+        ?ServiceProviderAggregateInterface $providers = null,
+        ?InflectorAggregateInterface $inflectors = null
     ) {
         $this->definitions = $definitions ?? new DefinitionAggregate();
         $this->providers   = $providers   ?? new ServiceProviderAggregate();
@@ -153,7 +153,7 @@ public function has($id): bool
         return false;
     }
 
-    public function inflector(string $type, callable $callback = null): InflectorInterface
+    public function inflector(string $type, ?callable $callback = null): InflectorInterface
     {
         return $this->inflectors->add($type, $callback);
     }
diff --git a/src/DefinitionContainerInterface.php b/src/DefinitionContainerInterface.php
index 7e7e57e..d7d5338 100644
--- a/src/DefinitionContainerInterface.php
+++ b/src/DefinitionContainerInterface.php
@@ -16,5 +16,5 @@ public function addServiceProvider(ServiceProviderInterface $provider): self;
     public function addShared(string $id, $concrete = null): DefinitionInterface;
     public function extend(string $id): DefinitionInterface;
     public function getNew($id);
-    public function inflector(string $type, callable $callback = null): InflectorInterface;
+    public function inflector(string $type, ?callable $callback = null): InflectorInterface;
 }
diff --git a/src/Inflector/Inflector.php b/src/Inflector/Inflector.php
index b8f0675..e0809cd 100644
--- a/src/Inflector/Inflector.php
+++ b/src/Inflector/Inflector.php
@@ -33,7 +33,7 @@ class Inflector implements ArgumentResolverInterface, InflectorInterface
      */
     protected $properties = [];
 
-    public function __construct(string $type, callable $callback = null)
+    public function __construct(string $type, ?callable $callback = null)
     {
         $this->type = $type;
         $this->callback = $callback;
diff --git a/src/Inflector/InflectorAggregate.php b/src/Inflector/InflectorAggregate.php
index c719332..70922d4 100644
--- a/src/Inflector/InflectorAggregate.php
+++ b/src/Inflector/InflectorAggregate.php
@@ -16,7 +16,7 @@ class InflectorAggregate implements InflectorAggregateInterface
      */
     protected $inflectors = [];
 
-    public function add(string $type, callable $callback = null): Inflector
+    public function add(string $type, ?callable $callback = null): Inflector
     {
         $inflector = new Inflector($type, $callback);
         $this->inflectors[] = $inflector;
diff --git a/src/Inflector/InflectorAggregateInterface.php b/src/Inflector/InflectorAggregateInterface.php
index 474276b..c95ac17 100644
--- a/src/Inflector/InflectorAggregateInterface.php
+++ b/src/Inflector/InflectorAggregateInterface.php
@@ -9,6 +9,6 @@
 
 interface InflectorAggregateInterface extends ContainerAwareInterface, IteratorAggregate
 {
-    public function add(string $type, callable $callback = null): Inflector;
+    public function add(string $type, ?callable $callback = null): Inflector;
     public function inflect(object $object);
 }
diff --git a/tests/Asset/Baz.php b/tests/Asset/Baz.php
index ff83441..e138299 100644
--- a/tests/Asset/Baz.php
+++ b/tests/Asset/Baz.php
@@ -6,7 +6,7 @@
 
 class Baz
 {
-    public function __construct(BarInterface $bar = null)
+    public function __construct(?BarInterface $bar = null)
     {
     }
 }
diff --git a/tests/Asset/Foo.php b/tests/Asset/Foo.php
index c7b6d6d..b624c5c 100644
--- a/tests/Asset/Foo.php
+++ b/tests/Asset/Foo.php
@@ -12,7 +12,7 @@ class Foo
 
     public static $staticHello;
 
-    public function __construct(Bar $bar = null)
+    public function __construct(?Bar $bar = null)
     {
         $this->bar = $bar;
     }