diff --git a/src/CodeCoverage/Generators/AbstractGenerator.php b/src/CodeCoverage/Generators/AbstractGenerator.php
index 8eaf34c6..671af88e 100644
--- a/src/CodeCoverage/Generators/AbstractGenerator.php
+++ b/src/CodeCoverage/Generators/AbstractGenerator.php
@@ -72,7 +72,7 @@ public function __construct(string $file, array $sources = [])
 	}
 
 
-	public function render(string $file = null): void
+	public function render(?string $file = null): void
 	{
 		$handle = $file ? @fopen($file, 'w') : STDOUT; // @ is escalated to exception
 		if (!$handle) {
diff --git a/src/CodeCoverage/Generators/CloverXMLGenerator.php b/src/CodeCoverage/Generators/CloverXMLGenerator.php
index b088a1d0..1e04d35b 100644
--- a/src/CodeCoverage/Generators/CloverXMLGenerator.php
+++ b/src/CodeCoverage/Generators/CloverXMLGenerator.php
@@ -151,7 +151,7 @@ protected function renderSelf(): void
 	}
 
 
-	private function calculateClassMetrics(\stdClass $info, array $coverageData = null): \stdClass
+	private function calculateClassMetrics(\stdClass $info, ?array $coverageData = null): \stdClass
 	{
 		$stats = (object) [
 			'methodCount' => count($info->methods),
@@ -182,7 +182,7 @@ private function calculateClassMetrics(\stdClass $info, array $coverageData = nu
 	}
 
 
-	private static function analyzeMethod(\stdClass $info, array $coverageData = null): array
+	private static function analyzeMethod(\stdClass $info, ?array $coverageData = null): array
 	{
 		$count = 0;
 		$coveredCount = 0;
diff --git a/src/CodeCoverage/Generators/HtmlGenerator.php b/src/CodeCoverage/Generators/HtmlGenerator.php
index ad02199e..2bf0412d 100644
--- a/src/CodeCoverage/Generators/HtmlGenerator.php
+++ b/src/CodeCoverage/Generators/HtmlGenerator.php
@@ -34,7 +34,7 @@ class HtmlGenerator extends AbstractGenerator
 	 * @param  string  $file  path to coverage.dat file
 	 * @param  array   $sources  files/directories
 	 */
-	public function __construct(string $file, array $sources = [], string $title = null)
+	public function __construct(string $file, array $sources = [], ?string $title = null)
 	{
 		parent::__construct($file, $sources);
 		$this->title = $title;
diff --git a/src/Framework/Assert.php b/src/Framework/Assert.php
index f6adc733..3ed4dc96 100644
--- a/src/Framework/Assert.php
+++ b/src/Framework/Assert.php
@@ -53,7 +53,7 @@ class Assert
 	/**
 	 * Asserts that two values are equal and have the same type and identity of objects.
 	 */
-	public static function same($expected, $actual, string $description = null): void
+	public static function same($expected, $actual, ?string $description = null): void
 	{
 		self::$counter++;
 		if ($actual !== $expected) {
@@ -65,7 +65,7 @@ public static function same($expected, $actual, string $description = null): voi
 	/**
 	 * Asserts that two values are not equal or do not have the same type and identity of objects.
 	 */
-	public static function notSame($expected, $actual, string $description = null): void
+	public static function notSame($expected, $actual, ?string $description = null): void
 	{
 		self::$counter++;
 		if ($actual === $expected) {
@@ -78,7 +78,7 @@ public static function notSame($expected, $actual, string $description = null):
 	 * Asserts that two values are equal and checks expectations. The identity of objects,
 	 * the order of keys in the arrays and marginally different floats are ignored.
 	 */
-	public static function equal($expected, $actual, string $description = null): void
+	public static function equal($expected, $actual, ?string $description = null): void
 	{
 		self::$counter++;
 		if (!self::isEqual($expected, $actual)) {
@@ -91,7 +91,7 @@ public static function equal($expected, $actual, string $description = null): vo
 	 * Asserts that two values are not equal and checks expectations. The identity of objects,
 	 * the order of keys in the arrays and marginally different floats are ignored.
 	 */
-	public static function notEqual($expected, $actual, string $description = null): void
+	public static function notEqual($expected, $actual, ?string $description = null): void
 	{
 		self::$counter++;
 		try {
@@ -110,7 +110,7 @@ public static function notEqual($expected, $actual, string $description = null):
 	 * @param  mixed  $needle
 	 * @param  array|string  $actual
 	 */
-	public static function contains($needle, $actual, string $description = null): void
+	public static function contains($needle, $actual, ?string $description = null): void
 	{
 		self::$counter++;
 		if (is_array($actual)) {
@@ -135,7 +135,7 @@ public static function contains($needle, $actual, string $description = null): v
 	 * @param  mixed  $needle
 	 * @param  array|string  $actual
 	 */
-	public static function notContains($needle, $actual, string $description = null): void
+	public static function notContains($needle, $actual, ?string $description = null): void
 	{
 		self::$counter++;
 		if (is_array($actual)) {
@@ -159,7 +159,7 @@ public static function notContains($needle, $actual, string $description = null)
 	 * Asserts that a haystack has an expected key.
 	 * @param  string|int  $key
 	 */
-	public static function hasKey($key, array $actual, string $description = null): void
+	public static function hasKey($key, array $actual, ?string $description = null): void
 	{
 		self::$counter++;
 		if (!is_int($key) && !is_string($key)) {
@@ -175,7 +175,7 @@ public static function hasKey($key, array $actual, string $description = null):
 	 * Asserts that a haystack doesn't have an expected key.
 	 * @param  string|int  $key
 	 */
-	public static function hasNotKey($key, array $actual, string $description = null): void
+	public static function hasNotKey($key, array $actual, ?string $description = null): void
 	{
 		self::$counter++;
 		if (!is_int($key) && !is_string($key)) {
@@ -191,7 +191,7 @@ public static function hasNotKey($key, array $actual, string $description = null
 	 * Asserts that a value is true.
 	 * @param  mixed  $actual
 	 */
-	public static function true($actual, string $description = null): void
+	public static function true($actual, ?string $description = null): void
 	{
 		self::$counter++;
 		if ($actual !== true) {
@@ -204,7 +204,7 @@ public static function true($actual, string $description = null): void
 	 * Asserts that a value is false.
 	 * @param  mixed  $actual
 	 */
-	public static function false($actual, string $description = null): void
+	public static function false($actual, ?string $description = null): void
 	{
 		self::$counter++;
 		if ($actual !== false) {
@@ -217,7 +217,7 @@ public static function false($actual, string $description = null): void
 	 * Asserts that a value is null.
 	 * @param  mixed  $actual
 	 */
-	public static function null($actual, string $description = null): void
+	public static function null($actual, ?string $description = null): void
 	{
 		self::$counter++;
 		if ($actual !== null) {
@@ -230,7 +230,7 @@ public static function null($actual, string $description = null): void
 	 * Asserts that a value is not null.
 	 * @param  mixed  $actual
 	 */
-	public static function notNull($actual, string $description = null): void
+	public static function notNull($actual, ?string $description = null): void
 	{
 		self::$counter++;
 		if ($actual === null) {
@@ -243,7 +243,7 @@ public static function notNull($actual, string $description = null): void
 	 * Asserts that a value is Not a Number.
 	 * @param  mixed  $actual
 	 */
-	public static function nan($actual, string $description = null): void
+	public static function nan($actual, ?string $description = null): void
 	{
 		self::$counter++;
 		if (!is_float($actual) || !is_nan($actual)) {
@@ -256,7 +256,7 @@ public static function nan($actual, string $description = null): void
 	 * Asserts that a value is truthy.
 	 * @param  mixed  $actual
 	 */
-	public static function truthy($actual, string $description = null): void
+	public static function truthy($actual, ?string $description = null): void
 	{
 		self::$counter++;
 		if (!$actual) {
@@ -269,7 +269,7 @@ public static function truthy($actual, string $description = null): void
 	 * Asserts that a value is falsey.
 	 * @param  mixed  $actual
 	 */
-	public static function falsey($actual, string $description = null): void
+	public static function falsey($actual, ?string $description = null): void
 	{
 		self::$counter++;
 		if ($actual) {
@@ -282,7 +282,7 @@ public static function falsey($actual, string $description = null): void
 	 * Asserts the number of items in an array or Countable.
 	 * @param  array|\Countable  $value
 	 */
-	public static function count(int $count, $value, string $description = null): void
+	public static function count(int $count, $value, ?string $description = null): void
 	{
 		self::$counter++;
 		if (!$value instanceof \Countable && !is_array($value)) {
@@ -299,7 +299,7 @@ public static function count(int $count, $value, string $description = null): vo
 	 * @param  string|object  $type
 	 * @param  mixed  $value
 	 */
-	public static function type($type, $value, string $description = null): void
+	public static function type($type, $value, ?string $description = null): void
 	{
 		self::$counter++;
 		if (!is_object($type) && !is_string($type)) {
@@ -329,7 +329,7 @@ public static function type($type, $value, string $description = null): void
 	public static function exception(
 		callable $function,
 		string $class,
-		string $message = null,
+		?string $message = null,
 		$code = null
 	): ?\Throwable {
 		self::$counter++;
@@ -359,7 +359,7 @@ public static function exception(
 	/**
 	 * Asserts that a function throws exception of given type and its message matches given pattern. Alias for exception().
 	 */
-	public static function throws(callable $function, string $class, string $message = null, $code = null): ?\Throwable
+	public static function throws(callable $function, string $class, ?string $message = null, $code = null): ?\Throwable
 	{
 		return self::exception($function, $class, $message, $code);
 	}
@@ -372,7 +372,7 @@ public static function throws(callable $function, string $class, string $message
 	 * @throws \Exception
 	 * @throws \Exception
 	 */
-	public static function error(callable $function, $expectedType, string $expectedMessage = null): ?\Throwable
+	public static function error(callable $function, $expectedType, ?string $expectedMessage = null): ?\Throwable
 	{
 		if (is_string($expectedType) && !preg_match('#^E_[A-Z_]+$#D', $expectedType)) {
 			return static::exception($function, $expectedType, $expectedMessage);
@@ -458,7 +458,7 @@ public static function noError(callable $function): void
 	 *   %h%    one or more HEX digits
 	 * @param  string  $pattern  mask|regexp; only delimiters ~ and # are supported for regexp
 	 */
-	public static function match(string $pattern, $actual, string $description = null): void
+	public static function match(string $pattern, $actual, ?string $description = null): void
 	{
 		self::$counter++;
 		if (!is_scalar($actual)) {
@@ -477,7 +477,7 @@ public static function match(string $pattern, $actual, string $description = nul
 	/**
 	 * Asserts that a string matches a given pattern stored in file.
 	 */
-	public static function matchFile(string $file, $actual, string $description = null): void
+	public static function matchFile(string $file, $actual, ?string $description = null): void
 	{
 		self::$counter++;
 		$pattern = @file_get_contents($file); // @ is escalated to exception
@@ -504,8 +504,8 @@ public static function fail(
 		string $message,
 		$actual = null,
 		$expected = null,
-		\Throwable $previous = null,
-		string $outputName = null
+		?\Throwable $previous = null,
+		?string $outputName = null
 	): void {
 		$e = new AssertException($message, $expected, $actual, $previous);
 		$e->outputName = $outputName;
@@ -517,7 +517,7 @@ public static function fail(
 	}
 
 
-	private static function describe(string $reason, string $description = null): string
+	private static function describe(string $reason, ?string $description = null): string
 	{
 		return ($description ? $description . ': ' : '') . $reason;
 	}
diff --git a/src/Framework/AssertException.php b/src/Framework/AssertException.php
index 4c8c9a5d..2a41829d 100644
--- a/src/Framework/AssertException.php
+++ b/src/Framework/AssertException.php
@@ -24,7 +24,7 @@ class AssertException extends \Exception
 	public $outputName;
 
 
-	public function __construct(string $message, $expected, $actual, \Throwable $previous = null)
+	public function __construct(string $message, $expected, $actual, ?\Throwable $previous = null)
 	{
 		parent::__construct('', 0, $previous);
 		$this->expected = $expected;
diff --git a/src/Framework/Dumper.php b/src/Framework/Dumper.php
index 658c78b6..02d764aa 100644
--- a/src/Framework/Dumper.php
+++ b/src/Framework/Dumper.php
@@ -418,7 +418,7 @@ public static function saveOutput(string $testFile, $content, string $suffix = '
 	/**
 	 * Applies color to string.
 	 */
-	public static function color(string $color = '', string $s = null): string
+	public static function color(string $color = '', ?string $s = null): string
 	{
 		static $colors = [
 			'black' => '0;30', 'gray' => '1;30', 'silver' => '0;37', 'white' => '1;37',
diff --git a/src/Framework/FileMock.php b/src/Framework/FileMock.php
index b8f827eb..fcf3bbdf 100644
--- a/src/Framework/FileMock.php
+++ b/src/Framework/FileMock.php
@@ -42,7 +42,7 @@ class FileMock
 	/**
 	 * @return string  file name
 	 */
-	public static function create(string $content = '', string $extension = null): string
+	public static function create(string $content = '', ?string $extension = null): string
 	{
 		self::register();
 
diff --git a/src/Framework/TestCase.php b/src/Framework/TestCase.php
index a65d7cca..a1e2671b 100644
--- a/src/Framework/TestCase.php
+++ b/src/Framework/TestCase.php
@@ -68,7 +68,7 @@ public function run(): void
 	 * Runs the test method.
 	 * @param  array  $args  test method parameters (dataprovider bypass)
 	 */
-	public function runTest(string $method, array $args = null): void
+	public function runTest(string $method, ?array $args = null): void
 	{
 		if (!method_exists($this, $method)) {
 			throw new TestCaseException("Method '$method' does not exist.");
diff --git a/src/Runner/CommandLine.php b/src/Runner/CommandLine.php
index 9574dc60..226e9137 100644
--- a/src/Runner/CommandLine.php
+++ b/src/Runner/CommandLine.php
@@ -71,7 +71,7 @@ public function __construct(string $help, array $defaults = [])
 	}
 
 
-	public function parse(array $args = null): array
+	public function parse(?array $args = null): array
 	{
 		if ($args === null) {
 			$args = isset($_SERVER['argv']) ? array_slice($_SERVER['argv'], 1) : [];
diff --git a/src/Runner/Job.php b/src/Runner/Job.php
index 62fdc7c5..9384633f 100644
--- a/src/Runner/Job.php
+++ b/src/Runner/Job.php
@@ -59,7 +59,7 @@ class Job
 	private $duration;
 
 
-	public function __construct(Test $test, PhpInterpreter $interpreter, array $envVars = null)
+	public function __construct(Test $test, PhpInterpreter $interpreter, ?array $envVars = null)
 	{
 		if ($test->getResult() !== Test::PREPARED) {
 			throw new \LogicException("Test '{$test->getSignature()}' already has result '{$test->getResult()}'.");
diff --git a/src/Runner/Output/ConsolePrinter.php b/src/Runner/Output/ConsolePrinter.php
index f10c943b..686ded10 100644
--- a/src/Runner/Output/ConsolePrinter.php
+++ b/src/Runner/Output/ConsolePrinter.php
@@ -51,7 +51,7 @@ class ConsolePrinter implements Tester\Runner\OutputHandler
 	public function __construct(
 		Runner $runner,
 		bool $displaySkipped = false,
-		string $file = null,
+		?string $file = null,
 		bool $ciderMode = false
 	) {
 		$this->runner = $runner;
diff --git a/src/Runner/Output/JUnitPrinter.php b/src/Runner/Output/JUnitPrinter.php
index 453a5a03..94b1d035 100644
--- a/src/Runner/Output/JUnitPrinter.php
+++ b/src/Runner/Output/JUnitPrinter.php
@@ -31,7 +31,7 @@ class JUnitPrinter implements Tester\Runner\OutputHandler
 	private $results;
 
 
-	public function __construct(string $file = null)
+	public function __construct(?string $file = null)
 	{
 		$this->file = fopen($file ?: 'php://output', 'w');
 	}
diff --git a/src/Runner/Output/Logger.php b/src/Runner/Output/Logger.php
index a7c29226..516826c9 100644
--- a/src/Runner/Output/Logger.php
+++ b/src/Runner/Output/Logger.php
@@ -32,7 +32,7 @@ class Logger implements Tester\Runner\OutputHandler
 	private $results;
 
 
-	public function __construct(Runner $runner, string $file = null)
+	public function __construct(Runner $runner, ?string $file = null)
 	{
 		$this->runner = $runner;
 		$this->file = fopen($file ?: 'php://output', 'w');
diff --git a/src/Runner/Output/TapPrinter.php b/src/Runner/Output/TapPrinter.php
index b03e1105..956ef3fb 100644
--- a/src/Runner/Output/TapPrinter.php
+++ b/src/Runner/Output/TapPrinter.php
@@ -25,7 +25,7 @@ class TapPrinter implements Tester\Runner\OutputHandler
 	private $results;
 
 
-	public function __construct(string $file = null)
+	public function __construct(?string $file = null)
 	{
 		$this->file = fopen($file ?: 'php://output', 'w');
 	}
diff --git a/src/Runner/PhpInterpreter.php b/src/Runner/PhpInterpreter.php
index ac160a77..c98e694d 100644
--- a/src/Runner/PhpInterpreter.php
+++ b/src/Runner/PhpInterpreter.php
@@ -89,7 +89,7 @@ public function __construct(string $path, array $args = [])
 	/**
 	 * @return static
 	 */
-	public function withPhpIniOption(string $name, string $value = null): self
+	public function withPhpIniOption(string $name, ?string $value = null): self
 	{
 		$me = clone $this;
 		$me->commandLine .= ' -d ' . Helpers::escapeArg($name . ($value === null ? '' : "=$value"));
diff --git a/src/Runner/Runner.php b/src/Runner/Runner.php
index eefe238c..96aac9f3 100644
--- a/src/Runner/Runner.php
+++ b/src/Runner/Runner.php
@@ -76,7 +76,7 @@ public function getEnvironmentVariables(): array
 	}
 
 
-	public function addPhpIniOption(string $name, string $value = null): void
+	public function addPhpIniOption(string $name, ?string $value = null): void
 	{
 		$this->interpreter = $this->interpreter->withPhpIniOption($name, $value);
 	}
diff --git a/src/Runner/Test.php b/src/Runner/Test.php
index 7825d0e1..85237aaa 100644
--- a/src/Runner/Test.php
+++ b/src/Runner/Test.php
@@ -46,7 +46,7 @@ class Test
 	private $args = [];
 
 
-	public function __construct(string $file, string $title = null)
+	public function __construct(string $file, ?string $title = null)
 	{
 		$this->file = $file;
 		$this->title = $title;
@@ -124,7 +124,7 @@ public function withArguments(array $args): self
 	/**
 	 * @return static
 	 */
-	public function withResult(int $result, ?string $message, float $duration = null): self
+	public function withResult(int $result, ?string $message, ?float $duration = null): self
 	{
 		if ($this->hasResult()) {
 			throw new \LogicException("Result of test is already set to $this->result with message '$this->message'.");