diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index f31b7dd..d0a4c07 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -13,7 +13,7 @@ jobs:
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with:
-          php-version: '8.0'
+          php-version: '7.1'
       - name: Install Composer Dependencies
         uses: ramsey/composer-install@v1
       - name: Run PHP-CS-Fixer
diff --git a/composer.json b/composer.json
index 9d3f070..96b2a9f 100644
--- a/composer.json
+++ b/composer.json
@@ -22,7 +22,7 @@
         "psr/http-server-middleware": "^1.0"
     },
     "require-dev": {
-        "doctrine/coding-standard": "^8.2",
+        "doctrine/coding-standard": "^9.0",
         "friendsofphp/php-cs-fixer": "^3.0",
         "laminas/laminas-diactoros": "~2.2",
         "phpstan/phpstan": "^0.12.14",
diff --git a/phpcs.xml b/phpcs.xml
index 678876e..1d6575d 100644
--- a/phpcs.xml
+++ b/phpcs.xml
@@ -19,8 +19,5 @@
         <exclude name="SlevomatCodingStandard.Functions.StaticClosure.ClosureNotStatic"/>
         <exclude name="SlevomatCodingStandard.TypeHints.TypeHintDeclaration.MissingReturnTypeHint"/>
         <exclude name="SlevomatCodingStandard.TypeHints.TypeHintDeclaration.MissingParameterTypeHint"/>
-        <exclude name="SlevomatCodingStandard.TypeHints.ReturnTypeHint.MissingNativeTypeHint"/>
-        <exclude name="SlevomatCodingStandard.TypeHints.ParameterTypeHint.MissingNativeTypeHint"/>
-        <exclude name="SlevomatCodingStandard.TypeHints.PropertyTypeHint.MissingNativeTypeHint"/>
     </rule>
 </ruleset>
diff --git a/src/RelayBuilder.php b/src/RelayBuilder.php
index ba8fc0e..7044854 100644
--- a/src/RelayBuilder.php
+++ b/src/RelayBuilder.php
@@ -10,13 +10,15 @@ class RelayBuilder
     /**
      * @param callable $resolver Converts a given queue entry to a callable or MiddlewareInterface instance.
      */
-    public function __construct(callable $resolver = null)
+    public function __construct(?callable $resolver = null)
     {
         $this->resolver = $resolver;
     }
 
     /**
      * @param iterable<mixed> $queue A queue of middleware entries.
+     *
+     * @phpcsSuppress SlevomatCodingStandard.TypeHints.ParameterTypeHint.MissingNativeTypeHint
      */
     public function newInstance($queue): Relay
     {
diff --git a/src/RequestHandler.php b/src/RequestHandler.php
index c1535cc..2b2e520 100644
--- a/src/RequestHandler.php
+++ b/src/RequestHandler.php
@@ -25,8 +25,10 @@ abstract class RequestHandler implements RequestHandlerInterface
     /**
      * @param iterable<mixed> $queue    A queue of middleware entries.
      * @param callable        $resolver Converts a given queue entry to a callable or MiddlewareInterface instance.
+     *
+     * @phpcsSuppress SlevomatCodingStandard.TypeHints.ParameterTypeHint.MissingNativeTypeHint
      */
-    public function __construct($queue, callable $resolver = null)
+    public function __construct($queue, ?callable $resolver = null)
     {
         if (! is_iterable($queue)) {
             throw new TypeError('\$queue must be array or Traversable.');
diff --git a/tests/RelayBuilderTest.php b/tests/RelayBuilderTest.php
index 2e4d27a..47e0f84 100644
--- a/tests/RelayBuilderTest.php
+++ b/tests/RelayBuilderTest.php
@@ -18,21 +18,21 @@ protected function setUp(): void
         $this->relayBuilder = new RelayBuilder();
     }
 
-    public function testArray()
+    public function testArray(): void
     {
         $queue = [new FakeMiddleware()];
         $relay = $this->relayBuilder->newInstance($queue);
         $this->assertInstanceOf('Relay\Relay', $relay);
     }
 
-    public function testArrayObject()
+    public function testArrayObject(): void
     {
         $queue = new ArrayObject([new FakeMiddleware()]);
         $relay = $this->relayBuilder->newInstance($queue);
         $this->assertInstanceOf('Relay\Relay', $relay);
     }
 
-    public function testTraversable()
+    public function testTraversable(): void
     {
         $queue = new class implements IteratorAggregate {
             public function getIterator(): Generator
@@ -48,13 +48,13 @@ public function getIterator(): Generator
     /**
      * @psalm-suppress InvalidArgument
      */
-    public function testInvalidArgument()
+    public function testInvalidArgument(): void
     {
         $this->expectException('TypeError');
         $this->relayBuilder->newInstance('bad argument');
     }
 
-    public function testEmptyQueue()
+    public function testEmptyQueue(): void
     {
         $this->expectException(InvalidArgumentException::class);
         $this->expectExceptionMessage('$queue cannot be empty');
diff --git a/tests/RelayTest.php b/tests/RelayTest.php
index 986c7c2..d4d591b 100644
--- a/tests/RelayTest.php
+++ b/tests/RelayTest.php
@@ -26,7 +26,7 @@ protected function setUp(): void
         };
     }
 
-    protected function assertRelay(Relay $relay)
+    protected function assertRelay(Relay $relay): void
     {
         FakeMiddleware::$count = 0;
 
@@ -41,7 +41,7 @@ protected function assertRelay(Relay $relay)
         $this->assertSame('<6<5<4', $actual);
     }
 
-    public function testArrayQueue()
+    public function testArrayQueue(): void
     {
         $queue = [
             new FakeMiddleware(),
@@ -53,7 +53,7 @@ public function testArrayQueue()
         $this->assertRelay(new Relay($queue));
     }
 
-    public function testTraversableQueue()
+    public function testTraversableQueue(): void
     {
         $queue = new class implements IteratorAggregate {
             public function getIterator(): Generator
@@ -73,13 +73,13 @@ public function getIterator(): Generator
     /**
      * @psalm-suppress InvalidArgument
      */
-    public function testBadQueue()
+    public function testBadQueue(): void
     {
         $this->expectException(TypeError::class);
         new Relay('bad');
     }
 
-    public function testEmptyQueue()
+    public function testEmptyQueue(): void
     {
         $this->expectException(InvalidArgumentException::class);
         $this->expectExceptionMessage('$queue cannot be empty');
@@ -87,7 +87,7 @@ public function testEmptyQueue()
         new Relay([]);
     }
 
-    public function testQueueWithInvalidEntry()
+    public function testQueueWithInvalidEntry(): void
     {
         $this->expectException(RuntimeException::class);
         $this->expectExceptionMessage(
@@ -98,7 +98,7 @@ public function testQueueWithInvalidEntry()
         $relay->handle(ServerRequestFactory::fromGlobals());
     }
 
-    public function testResolverEntries()
+    public function testResolverEntries(): void
     {
         $queue = [
             FakeMiddleware::class,
@@ -112,7 +112,7 @@ public function testResolverEntries()
         $this->assertRelay(new Relay($queue, $resolver));
     }
 
-    public function testRequestHandlerInQueue()
+    public function testRequestHandlerInQueue(): void
     {
         $queue          = [
             new FakeMiddleware(),
@@ -124,7 +124,7 @@ public function testRequestHandlerInQueue()
         $this->assertRelay(new Relay([$requestHandler]));
     }
 
-    public function testCallableMiddleware()
+    public function testCallableMiddleware(): void
     {
         $queue = [
             function (