diff --git a/Countries.php b/Countries.php
index fd17c3f0..256bd54e 100644
--- a/Countries.php
+++ b/Countries.php
@@ -127,7 +127,7 @@ public static function numericCodeExists(string $numericCode): bool
      *
      * @throws MissingResourceException if the country code does not exist
      */
-    public static function getName(string $country, string $displayLocale = null): string
+    public static function getName(string $country, ?string $displayLocale = null): string
     {
         return self::readEntry(['Names', $country], $displayLocale);
     }
@@ -137,7 +137,7 @@ public static function getName(string $country, string $displayLocale = null): s
      *
      * @throws MissingResourceException if the country code does not exist
      */
-    public static function getAlpha3Name(string $alpha3Code, string $displayLocale = null): string
+    public static function getAlpha3Name(string $alpha3Code, ?string $displayLocale = null): string
     {
         return self::getName(self::getAlpha2Code($alpha3Code), $displayLocale);
     }
@@ -147,7 +147,7 @@ public static function getAlpha3Name(string $alpha3Code, string $displayLocale =
      *
      * @return array<string, string>
      */
-    public static function getNames(string $displayLocale = null): array
+    public static function getNames(?string $displayLocale = null): array
     {
         return self::asort(self::readEntry(['Names'], $displayLocale), $displayLocale);
     }
@@ -159,7 +159,7 @@ public static function getNames(string $displayLocale = null): array
      *
      * @return array<string, string>
      */
-    public static function getAlpha3Names(string $displayLocale = null): array
+    public static function getAlpha3Names(?string $displayLocale = null): array
     {
         $alpha2Names = self::getNames($displayLocale);
         $alpha3Names = [];
diff --git a/Currencies.php b/Currencies.php
index 2f52576d..ce7fe5e2 100644
--- a/Currencies.php
+++ b/Currencies.php
@@ -50,7 +50,7 @@ public static function exists(string $currency): bool
     /**
      * @throws MissingResourceException if the currency code does not exist
      */
-    public static function getName(string $currency, string $displayLocale = null): string
+    public static function getName(string $currency, ?string $displayLocale = null): string
     {
         return self::readEntry(['Names', $currency, self::INDEX_NAME], $displayLocale);
     }
@@ -58,7 +58,7 @@ public static function getName(string $currency, string $displayLocale = null):
     /**
      * @return string[]
      */
-    public static function getNames(string $displayLocale = null): array
+    public static function getNames(?string $displayLocale = null): array
     {
         // ====================================================================
         // For reference: It is NOT possible to return names indexed by
@@ -82,7 +82,7 @@ public static function getNames(string $displayLocale = null): array
     /**
      * @throws MissingResourceException if the currency code does not exist
      */
-    public static function getSymbol(string $currency, string $displayLocale = null): string
+    public static function getSymbol(string $currency, ?string $displayLocale = null): string
     {
         return self::readEntry(['Names', $currency, self::INDEX_SYMBOL], $displayLocale);
     }
diff --git a/Data/Generator/TimezoneDataGenerator.php b/Data/Generator/TimezoneDataGenerator.php
index a0573e36..9de6a0cd 100644
--- a/Data/Generator/TimezoneDataGenerator.php
+++ b/Data/Generator/TimezoneDataGenerator.php
@@ -159,7 +159,7 @@ private function generateZones(BundleEntryReaderInterface $reader, string $tempD
 
         $regionFormat = $reader->readEntry($tempDir, $locale, ['zoneStrings', 'regionFormat']);
         $fallbackFormat = $reader->readEntry($tempDir, $locale, ['zoneStrings', 'fallbackFormat']);
-        $resolveName = function (string $id, string $city = null) use ($reader, $tempDir, $locale, $regionFormat, $fallbackFormat): ?string {
+        $resolveName = function (string $id, ?string $city = null) use ($reader, $tempDir, $locale, $regionFormat, $fallbackFormat): ?string {
             // Resolve default name as described per http://cldr.unicode.org/translation/timezones
             if (isset($this->zoneToCountryMapping[$id])) {
                 try {
diff --git a/Languages.php b/Languages.php
index e00b89fa..bff314b9 100644
--- a/Languages.php
+++ b/Languages.php
@@ -56,7 +56,7 @@ public static function exists(string $language): bool
      *
      * @throws MissingResourceException if the language code does not exist
      */
-    public static function getName(string $language, string $displayLocale = null): string
+    public static function getName(string $language, ?string $displayLocale = null): string
     {
         try {
             return self::readEntry(['Names', $language], $displayLocale);
@@ -78,7 +78,7 @@ public static function getName(string $language, string $displayLocale = null):
      *
      * @return array<string, string>
      */
-    public static function getNames(string $displayLocale = null): array
+    public static function getNames(?string $displayLocale = null): array
     {
         return self::asort(self::readEntry(['Names'], $displayLocale), $displayLocale);
     }
@@ -137,7 +137,7 @@ public static function alpha3CodeExists(string $language): bool
      *
      * @throws MissingResourceException if the country code does not exists
      */
-    public static function getAlpha3Name(string $language, string $displayLocale = null): string
+    public static function getAlpha3Name(string $language, ?string $displayLocale = null): string
     {
         try {
             return self::getName(self::getAlpha2Code($language), $displayLocale);
@@ -157,7 +157,7 @@ public static function getAlpha3Name(string $language, string $displayLocale = n
      *
      * @return array<string, string>
      */
-    public static function getAlpha3Names(string $displayLocale = null): array
+    public static function getAlpha3Names(?string $displayLocale = null): array
     {
         $alpha2Names = self::getNames($displayLocale);
         $alpha3Names = [];
diff --git a/Locales.php b/Locales.php
index 10d3dcef..ea4f50a7 100644
--- a/Locales.php
+++ b/Locales.php
@@ -51,7 +51,7 @@ public static function exists(string $locale): bool
     /**
      * @throws MissingResourceException if the locale does not exist
      */
-    public static function getName(string $locale, string $displayLocale = null): string
+    public static function getName(string $locale, ?string $displayLocale = null): string
     {
         try {
             return self::readEntry(['Names', $locale], $displayLocale);
@@ -67,7 +67,7 @@ public static function getName(string $locale, string $displayLocale = null): st
     /**
      * @return string[]
      */
-    public static function getNames(string $displayLocale = null): array
+    public static function getNames(?string $displayLocale = null): array
     {
         return self::asort(self::readEntry(['Names'], $displayLocale), $displayLocale);
     }
diff --git a/ResourceBundle.php b/ResourceBundle.php
index f1bc4825..be5017ce 100644
--- a/ResourceBundle.php
+++ b/ResourceBundle.php
@@ -43,7 +43,7 @@ abstract protected static function getPath(): string;
      * @return mixed returns an array or {@link \ArrayAccess} instance for
      *               complex data and a scalar value for simple data
      */
-    final protected static function readEntry(array $indices, string $locale = null, bool $fallback = true): mixed
+    final protected static function readEntry(array $indices, ?string $locale = null, bool $fallback = true): mixed
     {
         if (!isset(self::$entryReader)) {
             self::$entryReader = new BundleEntryReader(new BufferedBundleReader(
@@ -58,7 +58,7 @@ final protected static function readEntry(array $indices, string $locale = null,
         return self::$entryReader->readEntry(static::getPath(), $locale ?? \Locale::getDefault(), $indices, $fallback);
     }
 
-    final protected static function asort(iterable $list, string $locale = null): array
+    final protected static function asort(iterable $list, ?string $locale = null): array
     {
         if ($list instanceof \Traversable) {
             $list = iterator_to_array($list);
diff --git a/Scripts.php b/Scripts.php
index d21ed6fa..27e46067 100644
--- a/Scripts.php
+++ b/Scripts.php
@@ -43,7 +43,7 @@ public static function exists(string $script): bool
     /**
      * @throws MissingResourceException if the script code does not exist
      */
-    public static function getName(string $script, string $displayLocale = null): string
+    public static function getName(string $script, ?string $displayLocale = null): string
     {
         return self::readEntry(['Names', $script], $displayLocale);
     }
@@ -51,7 +51,7 @@ public static function getName(string $script, string $displayLocale = null): st
     /**
      * @return string[]
      */
-    public static function getNames(string $displayLocale = null): array
+    public static function getNames(?string $displayLocale = null): array
     {
         return self::asort(self::readEntry(['Names'], $displayLocale), $displayLocale);
     }
diff --git a/Timezones.php b/Timezones.php
index 0f9f4e3d..53d054f2 100644
--- a/Timezones.php
+++ b/Timezones.php
@@ -49,7 +49,7 @@ public static function exists(string $timezone): bool
     /**
      * @throws MissingResourceException if the timezone identifier does not exist or is an alias
      */
-    public static function getName(string $timezone, string $displayLocale = null): string
+    public static function getName(string $timezone, ?string $displayLocale = null): string
     {
         return self::readEntry(['Names', $timezone], $displayLocale);
     }
@@ -57,7 +57,7 @@ public static function getName(string $timezone, string $displayLocale = null):
     /**
      * @return string[]
      */
-    public static function getNames(string $displayLocale = null): array
+    public static function getNames(?string $displayLocale = null): array
     {
         return self::asort(self::readEntry(['Names'], $displayLocale), $displayLocale);
     }
@@ -66,14 +66,14 @@ public static function getNames(string $displayLocale = null): array
      * @throws \Exception       if the timezone identifier does not exist
      * @throws RuntimeException if there's no timezone DST transition information available
      */
-    public static function getRawOffset(string $timezone, int $timestamp = null): int
+    public static function getRawOffset(string $timezone, ?int $timestamp = null): int
     {
         $dateTimeImmutable = new \DateTimeImmutable(date('Y-m-d H:i:s', $timestamp ?? time()), new \DateTimeZone($timezone));
 
         return $dateTimeImmutable->getOffset();
     }
 
-    public static function getGmtOffset(string $timezone, int $timestamp = null, string $displayLocale = null): string
+    public static function getGmtOffset(string $timezone, ?int $timestamp = null, ?string $displayLocale = null): string
     {
         $offset = self::getRawOffset($timezone, $timestamp);
         $abs = abs($offset);
diff --git a/Util/GitRepository.php b/Util/GitRepository.php
index acbe8c49..1f8870cc 100644
--- a/Util/GitRepository.php
+++ b/Util/GitRepository.php
@@ -69,7 +69,7 @@ public function getLastAuthoredDate(): \DateTimeImmutable
         return new \DateTimeImmutable($this->getLastLine($this->execInPath('git log -1 --format="%ai"')));
     }
 
-    public function getLastTag(callable $filter = null): string
+    public function getLastTag(?callable $filter = null): string
     {
         $tags = $this->execInPath('git tag -l --sort=v:refname');
 
@@ -90,7 +90,7 @@ private function execInPath(string $command): array
         return self::exec(sprintf('cd %s && %s', escapeshellarg($this->path), $command));
     }
 
-    private static function exec(string $command, string $customErrorMessage = null): array
+    private static function exec(string $command, ?string $customErrorMessage = null): array
     {
         exec(sprintf('%s 2>&1', $command), $output, $result);
 
diff --git a/Util/IcuVersion.php b/Util/IcuVersion.php
index b75987f7..22c15ae2 100644
--- a/Util/IcuVersion.php
+++ b/Util/IcuVersion.php
@@ -48,7 +48,7 @@ class IcuVersion
      *
      * @see normalize()
      */
-    public static function compare(string $version1, string $version2, string $operator, int $precision = null): bool
+    public static function compare(string $version1, string $version2, string $operator, ?int $precision = null): bool
     {
         $version1 = self::normalize($version1, $precision);
         $version2 = self::normalize($version2, $precision);
diff --git a/Util/IntlTestHelper.php b/Util/IntlTestHelper.php
index f937f2eb..dd875f3c 100644
--- a/Util/IntlTestHelper.php
+++ b/Util/IntlTestHelper.php
@@ -30,7 +30,7 @@ class IntlTestHelper
     /**
      * Should be called before tests that work fine with the stub implementation.
      */
-    public static function requireIntl(TestCase $testCase, string $minimumIcuVersion = null): void
+    public static function requireIntl(TestCase $testCase, ?string $minimumIcuVersion = null): void
     {
         $minimumIcuVersion ??= Intl::getIcuStubVersion();
 
@@ -62,7 +62,7 @@ public static function requireIntl(TestCase $testCase, string $minimumIcuVersion
      * Should be called before tests that require a feature-complete intl
      * implementation.
      */
-    public static function requireFullIntl(TestCase $testCase, string $minimumIcuVersion = null): void
+    public static function requireFullIntl(TestCase $testCase, ?string $minimumIcuVersion = null): void
     {
         // We only run tests if the intl extension is loaded...
         if (!Intl::isExtensionLoaded()) {
diff --git a/Util/Version.php b/Util/Version.php
index 8d72c3af..f30709ec 100644
--- a/Util/Version.php
+++ b/Util/Version.php
@@ -38,7 +38,7 @@ class Version
      *
      * @see normalize()
      */
-    public static function compare(string $version1, string $version2, string $operator, int $precision = null): bool
+    public static function compare(string $version1, string $version2, string $operator, ?int $precision = null): bool
     {
         $version1 = self::normalize($version1, $precision);
         $version2 = self::normalize($version2, $precision);