diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 05e7b40..3584e6c 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -31,4 +31,4 @@ jobs:
       os: >-
         ['ubuntu-latest', 'windows-latest']
       php: >-
-        ['8.1', '8.2', '8.3']
+        ['8.1', '8.2', '8.3', '8.4']
diff --git a/.github/workflows/composer-require-checker.yml b/.github/workflows/composer-require-checker.yml
index a857bce..a93390b 100644
--- a/.github/workflows/composer-require-checker.yml
+++ b/.github/workflows/composer-require-checker.yml
@@ -31,4 +31,4 @@ jobs:
       os: >-
         ['ubuntu-latest']
       php: >-
-        ['8.1', '8.2', '8.3']
+        ['8.1', '8.2', '8.3', '8.4']
diff --git a/.github/workflows/rector.yml b/.github/workflows/rector.yml
index 457772a..5d6931d 100644
--- a/.github/workflows/rector.yml
+++ b/.github/workflows/rector.yml
@@ -21,4 +21,4 @@ jobs:
       os: >-
         ['ubuntu-latest']
       php: >-
-        ['8.3']
+        ['8.4']
diff --git a/.github/workflows/static.yml b/.github/workflows/static.yml
index e33eca8..d03874d 100644
--- a/.github/workflows/static.yml
+++ b/.github/workflows/static.yml
@@ -29,4 +29,4 @@ jobs:
       os: >-
         ['ubuntu-latest']
       php: >-
-        ['8.1', '8.2', '8.3']
+        ['8.1', '8.2', '8.3', '8.4']
diff --git a/CHANGELOG.md b/CHANGELOG.md
index d294eec..1ccf578 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,7 +8,9 @@
   class (@olegbaturin)
 - Chg #137: Add separate parameters for each of `HtmlRenderer` settings in constructor. Mark `$settings` parameter as
   deprecated (@vjik) 
-- Enh #138: Raise the minimum PHP version to 8.1 and minor refactoring (@vjik) 
+- Enh #138, #139: Raise the minimum PHP version to 8.1 and minor refactoring (@vjik)
+- Chg #139: Change PHP constraint in `composer.json` to `~8.1.0 || ~8.2.0 || ~8.3.0 || ~8.4.0` (@vjik)
+- Bug #139: Explicitly mark nullable parameters (@vjik)
 
 ## 3.3.0 July 11, 2024
 
diff --git a/composer.json b/composer.json
index 02d53c4..0ec520e 100644
--- a/composer.json
+++ b/composer.json
@@ -31,7 +31,7 @@
         }
     ],
     "require": {
-        "php": "^8.1",
+        "php": "~8.1.0 || ~8.2.0 || ~8.3.0 || ~8.4.0",
         "ext-dom": "*",
         "ext-mbstring": "*",
         "alexkart/curl-builder": "^1.0",
@@ -47,14 +47,14 @@
         "yiisoft/injector": "^1.0"
     },
     "require-dev": {
+        "bamarni/composer-bin-plugin": "^1.8",
         "httpsoft/http-message": "^1.1.6",
-        "maglnet/composer-require-checker": "^4.7.1",
         "phpunit/phpunit": "^10.5.44",
         "psr/event-dispatcher": "^1.0",
         "rector/rector": "^2.0.7",
         "roave/infection-static-analysis-plugin": "^1.35",
         "spatie/phpunit-watcher": "^1.24",
-        "vimeo/psalm": "^5.26.1|^6",
+        "vimeo/psalm": "^5.26.1|^6.2",
         "yiisoft/di": "^1.3",
         "yiisoft/test-support": "^3.0.1"
     },
@@ -69,6 +69,11 @@
         }
     },
     "extra": {
+        "bamarni-bin": {
+            "bin-links": true,
+            "target-directory": "tools",
+            "forward-command": true
+        },
         "config-plugin-options": {
             "source-directory": "config"
         },
@@ -80,8 +85,9 @@
         "sort-packages": true,
         "bump-after-update": "dev",
         "allow-plugins": {
-            "infection/extension-installer": true,
-            "composer/package-versions-deprecated": true
+            "bamarni/composer-bin-plugin": true,
+            "composer/package-versions-deprecated": true,
+            "infection/extension-installer": true
         }
     },
     "scripts": {
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index ea84752..22ae7b7 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -9,6 +9,7 @@
          executionOrder="random"
          failOnRisky="true"
          failOnWarning="true"
+         failOnDeprecation="true"
          stopOnFailure="false"
          colors="true"
          displayDetailsOnPhpunitDeprecations="true"
diff --git a/rector.php b/rector.php
index 75ff290..6589abb 100644
--- a/rector.php
+++ b/rector.php
@@ -6,6 +6,7 @@
 use Rector\Config\RectorConfig;
 use Rector\Php71\Rector\FuncCall\RemoveExtraParametersRector;
 use Rector\Php74\Rector\Closure\ClosureToArrowFunctionRector;
+use Rector\Php81\Rector\ClassMethod\NewInInitializerRector;
 use Rector\Php81\Rector\FuncCall\NullToStrictStringFuncCallArgRector;
 
 return RectorConfig::configure()
@@ -21,4 +22,5 @@
         ClosureToArrowFunctionRector::class,
         NullToStrictStringFuncCallArgRector::class,
         RemoveExtraParametersRector::class,
+        NewInInitializerRector::class,
     ]);
diff --git a/src/ErrorHandler.php b/src/ErrorHandler.php
index b3d9ae0..f9c4aa0 100644
--- a/src/ErrorHandler.php
+++ b/src/ErrorHandler.php
@@ -62,8 +62,8 @@ public function __construct(
      */
     public function handle(
         Throwable $t,
-        ThrowableRendererInterface $renderer = null,
-        ServerRequestInterface $request = null
+        ?ThrowableRendererInterface $renderer = null,
+        ?ServerRequestInterface $request = null
     ): ErrorData {
         $renderer ??= $this->defaultRenderer;
 
@@ -192,6 +192,9 @@ private function initializeOnce(): void
         });
 
         if (!(PHP_SAPI === 'cli' || PHP_SAPI === 'phpdbg')) {
+            /**
+             * @var string
+             */
             $this->workingDirectory = getcwd();
         }
 
diff --git a/src/Exception/ErrorException.php b/src/Exception/ErrorException.php
index d2b98f1..11c1465 100644
--- a/src/Exception/ErrorException.php
+++ b/src/Exception/ErrorException.php
@@ -33,14 +33,14 @@ class ErrorException extends \ErrorException implements FriendlyExceptionInterfa
         E_USER_ERROR => 'PHP User Error',
         E_USER_WARNING => 'PHP User Warning',
         E_USER_NOTICE => 'PHP User Notice',
-        E_STRICT => 'PHP Strict Warning',
+        2048 => 'PHP Strict Warning', // E_STRICT
         E_RECOVERABLE_ERROR => 'PHP Recoverable Error',
         E_DEPRECATED => 'PHP Deprecated Warning',
         E_USER_DEPRECATED => 'PHP User Deprecated Warning',
     ];
 
     /** @psalm-param DebugBacktraceType $backtrace */
-    public function __construct(string $message = '', int $code = 0, int $severity = 1, string $filename = __FILE__, int $line = __LINE__, Exception $previous = null, private readonly array $backtrace = [])
+    public function __construct(string $message = '', int $code = 0, int $severity = 1, string $filename = __FILE__, int $line = __LINE__, ?Exception $previous = null, private readonly array $backtrace = [])
     {
         parent::__construct($message, $code, $severity, $filename, $line, $previous);
         $this->addXDebugTraceToFatalIfAvailable();
@@ -149,6 +149,6 @@ private function isXdebugStackAvailable(): bool
         }
 
         // Xdebug 3 and later, proper mode is required
-        return str_contains(ini_get('xdebug.mode'), 'develop');
+        return str_contains((string) \ini_get('xdebug.mode'), 'develop');
     }
 }
diff --git a/src/Factory/ThrowableResponseFactory.php b/src/Factory/ThrowableResponseFactory.php
index dd9db73..6daf9ac 100644
--- a/src/Factory/ThrowableResponseFactory.php
+++ b/src/Factory/ThrowableResponseFactory.php
@@ -53,10 +53,10 @@ final class ThrowableResponseFactory implements ThrowableResponseFactoryInterfac
     private ?string $contentType = null;
 
     public function __construct(
-        private ResponseFactoryInterface $responseFactory,
-        private ErrorHandler $errorHandler,
-        private ContainerInterface $container,
-        HeadersProvider $headersProvider = null,
+        private readonly ResponseFactoryInterface $responseFactory,
+        private readonly ErrorHandler $errorHandler,
+        private readonly ContainerInterface $container,
+        ?HeadersProvider $headersProvider = null,
     ) {
         $this->headersProvider = $headersProvider ?? new HeadersProvider();
     }
diff --git a/src/Renderer/HeaderRenderer.php b/src/Renderer/HeaderRenderer.php
index 0270024..385551b 100644
--- a/src/Renderer/HeaderRenderer.php
+++ b/src/Renderer/HeaderRenderer.php
@@ -14,12 +14,12 @@
  */
 final class HeaderRenderer implements ThrowableRendererInterface
 {
-    public function render(Throwable $t, ServerRequestInterface $request = null): ErrorData
+    public function render(Throwable $t, ?ServerRequestInterface $request = null): ErrorData
     {
         return new ErrorData('', ['X-Error-Message' => self::DEFAULT_ERROR_MESSAGE]);
     }
 
-    public function renderVerbose(Throwable $t, ServerRequestInterface $request = null): ErrorData
+    public function renderVerbose(Throwable $t, ?ServerRequestInterface $request = null): ErrorData
     {
         return new ErrorData('', [
             'X-Error-Type' => $t::class,
diff --git a/src/Renderer/HtmlRenderer.php b/src/Renderer/HtmlRenderer.php
index 9449187..9e8fc75 100644
--- a/src/Renderer/HtmlRenderer.php
+++ b/src/Renderer/HtmlRenderer.php
@@ -156,7 +156,7 @@ public function __construct(
             ?? null;
     }
 
-    public function render(Throwable $t, ServerRequestInterface $request = null): ErrorData
+    public function render(Throwable $t, ?ServerRequestInterface $request = null): ErrorData
     {
         return new ErrorData($this->renderTemplate($this->template, [
             'request' => $request,
@@ -164,7 +164,7 @@ public function render(Throwable $t, ServerRequestInterface $request = null): Er
         ]));
     }
 
-    public function renderVerbose(Throwable $t, ServerRequestInterface $request = null): ErrorData
+    public function renderVerbose(Throwable $t, ?ServerRequestInterface $request = null): ErrorData
     {
         return new ErrorData($this->renderTemplate($this->verboseTemplate, [
             'request' => $request,
@@ -530,7 +530,7 @@ private function renderTemplate(string $path, array $parameters): string
         try {
             /** @psalm-suppress PossiblyNullFunctionCall */
             $renderer->bindTo($this)($path, $parameters);
-            return ob_get_clean();
+            return (string) ob_get_clean();
         } catch (Throwable $e) {
             while (ob_get_level() > $obInitialLevel) {
                 if (!@ob_end_clean()) {
diff --git a/src/Renderer/JsonRenderer.php b/src/Renderer/JsonRenderer.php
index ef53296..028084e 100644
--- a/src/Renderer/JsonRenderer.php
+++ b/src/Renderer/JsonRenderer.php
@@ -16,7 +16,7 @@
  */
 final class JsonRenderer implements ThrowableRendererInterface
 {
-    public function render(Throwable $t, ServerRequestInterface $request = null): ErrorData
+    public function render(Throwable $t, ?ServerRequestInterface $request = null): ErrorData
     {
         return new ErrorData(
             json_encode(
@@ -28,7 +28,7 @@ public function render(Throwable $t, ServerRequestInterface $request = null): Er
         );
     }
 
-    public function renderVerbose(Throwable $t, ServerRequestInterface $request = null): ErrorData
+    public function renderVerbose(Throwable $t, ?ServerRequestInterface $request = null): ErrorData
     {
         return new ErrorData(
             json_encode(
@@ -40,7 +40,7 @@ public function renderVerbose(Throwable $t, ServerRequestInterface $request = nu
                     'line' => $t->getLine(),
                     'trace' => $t->getTrace(),
                 ],
-                JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES | JSON_INVALID_UTF8_SUBSTITUTE | JSON_PARTIAL_OUTPUT_ON_ERROR
+                JSON_THROW_ON_ERROR | JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES | JSON_INVALID_UTF8_SUBSTITUTE | JSON_PARTIAL_OUTPUT_ON_ERROR
             )
         );
     }
diff --git a/src/Renderer/PlainTextRenderer.php b/src/Renderer/PlainTextRenderer.php
index 76fc39a..1f9ddd5 100644
--- a/src/Renderer/PlainTextRenderer.php
+++ b/src/Renderer/PlainTextRenderer.php
@@ -14,12 +14,12 @@
  */
 final class PlainTextRenderer implements ThrowableRendererInterface
 {
-    public function render(Throwable $t, ServerRequestInterface $request = null): ErrorData
+    public function render(Throwable $t, ?ServerRequestInterface $request = null): ErrorData
     {
         return new ErrorData(self::DEFAULT_ERROR_MESSAGE);
     }
 
-    public function renderVerbose(Throwable $t, ServerRequestInterface $request = null): ErrorData
+    public function renderVerbose(Throwable $t, ?ServerRequestInterface $request = null): ErrorData
     {
         return new ErrorData(
             self::throwableToString($t)
diff --git a/src/Renderer/XmlRenderer.php b/src/Renderer/XmlRenderer.php
index 2bd63f2..e7af2cd 100644
--- a/src/Renderer/XmlRenderer.php
+++ b/src/Renderer/XmlRenderer.php
@@ -16,7 +16,7 @@
  */
 final class XmlRenderer implements ThrowableRendererInterface
 {
-    public function render(Throwable $t, ServerRequestInterface $request = null): ErrorData
+    public function render(Throwable $t, ?ServerRequestInterface $request = null): ErrorData
     {
         $content = '<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>';
         $content .= "\n<error>\n";
@@ -25,7 +25,7 @@ public function render(Throwable $t, ServerRequestInterface $request = null): Er
         return new ErrorData($content);
     }
 
-    public function renderVerbose(Throwable $t, ServerRequestInterface $request = null): ErrorData
+    public function renderVerbose(Throwable $t, ?ServerRequestInterface $request = null): ErrorData
     {
         $content = '<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>';
         $content .= "\n<error>\n";
diff --git a/src/ThrowableRendererInterface.php b/src/ThrowableRendererInterface.php
index a409a04..b6f4886 100644
--- a/src/ThrowableRendererInterface.php
+++ b/src/ThrowableRendererInterface.php
@@ -23,7 +23,7 @@ interface ThrowableRendererInterface
      * @param ServerRequestInterface|null $request
      * @return ErrorData
      */
-    public function render(Throwable $t, ServerRequestInterface $request = null): ErrorData;
+    public function render(Throwable $t, ?ServerRequestInterface $request = null): ErrorData;
 
     /**
      * Returns error data suitable for adding it to response in development environment.
@@ -31,5 +31,5 @@ public function render(Throwable $t, ServerRequestInterface $request = null): Er
      * @param ServerRequestInterface|null $request
      * @return ErrorData
      */
-    public function renderVerbose(Throwable $t, ServerRequestInterface $request = null): ErrorData;
+    public function renderVerbose(Throwable $t, ?ServerRequestInterface $request = null): ErrorData;
 }
diff --git a/tests/Factory/ThrowableResponseFactoryTest.php b/tests/Factory/ThrowableResponseFactoryTest.php
index df40fe2..6f6a239 100644
--- a/tests/Factory/ThrowableResponseFactoryTest.php
+++ b/tests/Factory/ThrowableResponseFactoryTest.php
@@ -233,7 +233,7 @@ public function testAddedHeaders(): void
     }
 
     private function createThrowableResponseFactory(
-        HeadersProvider $provider = null,
+        ?HeadersProvider $provider = null,
     ): ThrowableResponseFactoryInterface {
         $container = new SimpleContainer([], fn (string $className): object => new $className());
         return new ThrowableResponseFactory(
diff --git a/tests/Renderer/HtmlRendererTest.php b/tests/Renderer/HtmlRendererTest.php
index aa8026b..6b1308e 100644
--- a/tests/Renderer/HtmlRendererTest.php
+++ b/tests/Renderer/HtmlRendererTest.php
@@ -7,6 +7,7 @@
 use Exception;
 use HttpSoft\Message\Uri;
 use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\Attributes\WithoutErrorHandler;
 use PHPUnit\Framework\TestCase;
 use Psr\Http\Message\ServerRequestInterface;
 use ReflectionClass;
@@ -131,9 +132,16 @@ public function testRenderCallStack(): void
         );
     }
 
+    #[WithoutErrorHandler]
     public function testRenderCallStackItemIfFileIsNotExistAndLineMoreZero(): void
     {
-        $this->assertEmpty($this->invokeMethod(new HtmlRenderer(), 'renderCallStackItem', [
+        $errorMessage = null;
+        set_error_handler(
+            static function (int $code, string $message) use (&$errorMessage) {
+                $errorMessage = $message;
+            }
+        );
+        $result = $this->invokeMethod(new HtmlRenderer(), 'renderCallStackItem', [
             'file' => 'not-exist',
             'line' => 1,
             'class' => null,
@@ -142,7 +150,11 @@ public function testRenderCallStackItemIfFileIsNotExistAndLineMoreZero(): void
             'index' => 1,
             'isVendorFile' => false,
             'reflectionParameters' => [],
-        ]));
+        ]);
+        restore_error_handler();
+
+        $this->assertSame('', $result);
+        $this->assertSame('file(not-exist): Failed to open stream: No such file or directory', $errorMessage);
     }
 
     public function testRenderRequest(): void
diff --git a/tools/.gitignore b/tools/.gitignore
new file mode 100644
index 0000000..cf452dc
--- /dev/null
+++ b/tools/.gitignore
@@ -0,0 +1,2 @@
+/*/vendor
+/*/composer.lock
diff --git a/tools/composer-require-checker/composer.json b/tools/composer-require-checker/composer.json
new file mode 100644
index 0000000..35b552f
--- /dev/null
+++ b/tools/composer-require-checker/composer.json
@@ -0,0 +1,8 @@
+{
+    "require-dev": {
+        "maglnet/composer-require-checker": "^4.7.1"
+    },
+    "config": {
+        "bump-after-update": "dev"
+    }
+}