diff --git a/phpunit.xml b/phpunit.xml
index c8fdf32..441720c 100644
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -9,7 +9,6 @@
          processIsolation="false"
          stopOnError="false"
          stopOnFailure="false"
-         syntaxCheck="true"
          verbose="true"
         >
     <testsuites>
diff --git a/src/Busses/SynchronousCommandBus.php b/src/Busses/SynchronousCommandBus.php
index 4aacb83..acf3e0e 100644
--- a/src/Busses/SynchronousCommandBus.php
+++ b/src/Busses/SynchronousCommandBus.php
@@ -11,7 +11,7 @@ class SynchronousCommandBus implements CommandBus
 {
     protected $resolver;
 
-    public function __construct(CommandHandlerResolver $resolver = null)
+    public function __construct(?CommandHandlerResolver $resolver = null)
     {
         $this->resolver = $resolver ?: new NativeCommandHandlerResolver;
     }
diff --git a/src/Chief.php b/src/Chief.php
index 52d05f5..c56e19f 100644
--- a/src/Chief.php
+++ b/src/Chief.php
@@ -22,7 +22,7 @@ class Chief implements CommandBus
      * @param CommandBus $bus
      * @param array $decorators Array of \Chief\Decorator objects
      */
-    public function __construct(CommandBus $bus = null, array $decorators = [])
+    public function __construct(?CommandBus $bus = null, array $decorators = [])
     {
         $this->bus = $bus ?: new SynchronousCommandBus;
 
diff --git a/src/Decorators/CachingDecorator.php b/src/Decorators/CachingDecorator.php
index fbaf15c..57c3374 100644
--- a/src/Decorators/CachingDecorator.php
+++ b/src/Decorators/CachingDecorator.php
@@ -32,7 +32,7 @@ class CachingDecorator implements Decorator
      * @param int $expiresAfter
      * @param CommandBus $innerCommandBus
      */
-    public function __construct(CacheItemPoolInterface $cache, $expiresAfter = 3600, CommandBus $innerCommandBus = null)
+    public function __construct(CacheItemPoolInterface $cache, int $expiresAfter = 3600, ?CommandBus $innerCommandBus = null)
     {
         $this->cache = $cache;
         $this->expiresAfter = $expiresAfter;
diff --git a/src/Decorators/CommandQueueingDecorator.php b/src/Decorators/CommandQueueingDecorator.php
index cfc1eca..e83101a 100644
--- a/src/Decorators/CommandQueueingDecorator.php
+++ b/src/Decorators/CommandQueueingDecorator.php
@@ -25,7 +25,7 @@ class CommandQueueingDecorator implements Decorator
      * @param CommandQueuer $queuer
      * @param CommandBus $innerCommandBus
      */
-    public function __construct(CommandQueuer $queuer, CommandBus $innerCommandBus = null)
+    public function __construct(CommandQueuer $queuer, ?CommandBus $innerCommandBus = null)
     {
         $this->queuer = $queuer;
         $this->setInnerBus($innerCommandBus ?: new SynchronousCommandBus());
diff --git a/src/Decorators/EventDispatchingDecorator.php b/src/Decorators/EventDispatchingDecorator.php
index f3344ee..010e74c 100644
--- a/src/Decorators/EventDispatchingDecorator.php
+++ b/src/Decorators/EventDispatchingDecorator.php
@@ -22,9 +22,9 @@ class EventDispatchingDecorator implements Decorator
 
     /**
      * @param EventDispatcher $dispatcher
-     * @param CommandBus $innerCommandBus
+     * @param CommandBus|null $innerCommandBus
      */
-    public function __construct(EventDispatcher $dispatcher, CommandBus $innerCommandBus = null)
+    public function __construct(EventDispatcher $dispatcher, ?CommandBus $innerCommandBus = null)
     {
         $this->dispatcher = $dispatcher;
         $this->setInnerBus($innerCommandBus ?: new SynchronousCommandBus());
diff --git a/src/Decorators/LoggingDecorator.php b/src/Decorators/LoggingDecorator.php
index 21a715f..faf1417 100644
--- a/src/Decorators/LoggingDecorator.php
+++ b/src/Decorators/LoggingDecorator.php
@@ -27,7 +27,7 @@ class LoggingDecorator implements Decorator
      * the command execution, such as the request/session information.
      * @param CommandBus $innerCommandBus
      */
-    public function __construct(LoggerInterface $logger, $context = null, CommandBus $innerCommandBus = null)
+    public function __construct(LoggerInterface $logger, mixed $context = null, ?CommandBus $innerCommandBus = null)
     {
         $this->logger = $logger;
         $this->context = $context;
diff --git a/src/Decorators/TransactionalCommandLockingDecorator.php b/src/Decorators/TransactionalCommandLockingDecorator.php
index 57f8273..d8215ee 100644
--- a/src/Decorators/TransactionalCommandLockingDecorator.php
+++ b/src/Decorators/TransactionalCommandLockingDecorator.php
@@ -28,7 +28,7 @@ class TransactionalCommandLockingDecorator implements Decorator
      */
     protected $queue = [];
 
-    public function __construct(CommandBus $innerCommandBus = null)
+    public function __construct(?CommandBus $innerCommandBus = null)
     {
         $this->setInnerBus($innerCommandBus ?: new SynchronousCommandBus());
     }
diff --git a/src/Resolvers/NativeCommandHandlerResolver.php b/src/Resolvers/NativeCommandHandlerResolver.php
index c6ba902..29defdc 100644
--- a/src/Resolvers/NativeCommandHandlerResolver.php
+++ b/src/Resolvers/NativeCommandHandlerResolver.php
@@ -17,7 +17,7 @@ class NativeCommandHandlerResolver implements CommandHandlerResolver
 
     protected $handlers = [];
 
-    public function __construct(Container $container = null)
+    public function __construct(?Container $container = null)
     {
         $this->container = $container ?: new NativeContainer;
     }
diff --git a/tests/Decorators/CachingDecoratorTest.php b/tests/Decorators/CachingDecoratorTest.php
index d9bfff1..6c1753a 100644
--- a/tests/Decorators/CachingDecoratorTest.php
+++ b/tests/Decorators/CachingDecoratorTest.php
@@ -153,12 +153,22 @@ protected function getDecorator()
 
 class FakeCachableCommand implements CacheableCommand
 {
-    public $data;
+    private $data;
 
     public function __construct($data)
     {
         $this->data = $data;
     }
+
+    public function getData()
+    {
+        return $this->data;
+    }
+
+    public function setData($data)
+    {
+        $this->data = $data;
+    }
 }
 
 class FakeCachableCommandWithCacheOptions extends FakeCachableCommand implements HasCacheOptions
@@ -168,7 +178,7 @@ class FakeCachableCommandWithCacheOptions extends FakeCachableCommand implements
 
     public function __construct($data, $expiry, $cacheKey)
     {
-        $this->data = $data;
+        parent::__construct($data);
         $this->expiry = $expiry;
         $this->cacheKey = $cacheKey;
     }
diff --git a/tests/Stubs/LogDecoratorCommandBus.php b/tests/Stubs/LogDecoratorCommandBus.php
index faf0497..35fbb86 100644
--- a/tests/Stubs/LogDecoratorCommandBus.php
+++ b/tests/Stubs/LogDecoratorCommandBus.php
@@ -9,6 +9,9 @@
 
 class LogDecoratorCommandBus implements Decorator
 {
+    public $logger;
+    public $commandBus;
+
     public function __construct(LoggerInterface $logger, CommandBus $commandBus)
     {
         $this->logger = $logger;
diff --git a/tests/Stubs/NonInterfaceImplementingCommand.php b/tests/Stubs/NonInterfaceImplementingCommand.php
index 1b8330b..747d285 100644
--- a/tests/Stubs/NonInterfaceImplementingCommand.php
+++ b/tests/Stubs/NonInterfaceImplementingCommand.php
@@ -4,4 +4,6 @@
 
 use Chief\Command;
 
-class NonInterfaceImplementingCommand implements Command {}
\ No newline at end of file
+class NonInterfaceImplementingCommand implements Command {
+    public $handled = false;
+}
\ No newline at end of file
diff --git a/tests/Stubs/NonInterfaceImplementingCommandHandler.php b/tests/Stubs/NonInterfaceImplementingCommandHandler.php
index dccfc47..fc4e510 100644
--- a/tests/Stubs/NonInterfaceImplementingCommandHandler.php
+++ b/tests/Stubs/NonInterfaceImplementingCommandHandler.php
@@ -7,6 +7,8 @@
 
 class NonInterfaceImplementingCommandHandler
 {
+    public $handled = false;
+
     public function handle($command)
     {
         $command->handled = true;
diff --git a/tests/Stubs/SelfHandlingCommand.php b/tests/Stubs/SelfHandlingCommand.php
index 5fafb5b..35de6c8 100644
--- a/tests/Stubs/SelfHandlingCommand.php
+++ b/tests/Stubs/SelfHandlingCommand.php
@@ -7,6 +7,8 @@
 
 class SelfHandlingCommand implements Command, CommandHandler
 {
+    public $handled = false;
+
     public function handle(Command $command)
     {
         $command->handled = true;
diff --git a/tests/Stubs/TestCommand.php b/tests/Stubs/TestCommand.php
index 7e88caa..69a0b90 100644
--- a/tests/Stubs/TestCommand.php
+++ b/tests/Stubs/TestCommand.php
@@ -4,4 +4,6 @@
 
 use Chief\Command;
 
-class TestCommand implements Command {}
\ No newline at end of file
+class TestCommand implements Command {
+    public $handled = false;
+}
\ No newline at end of file