diff --git a/src/CurrencyConverter.php b/src/CurrencyConverter.php
index 73ef6f4..5ed7cde 100644
--- a/src/CurrencyConverter.php
+++ b/src/CurrencyConverter.php
@@ -39,7 +39,7 @@ final class CurrencyConverter
      * @param Context|null         $context              A context to create the monies in, or null to use the default.
      *                                                   The context only applies to convert(), not convertToRational().
      */
-    public function __construct(ExchangeRateProvider $exchangeRateProvider, Context $context = null)
+    public function __construct(ExchangeRateProvider $exchangeRateProvider, ?Context $context = null)
     {
         if ($context === null) {
             $context = new DefaultContext();
diff --git a/src/Exception/CurrencyConversionException.php b/src/Exception/CurrencyConversionException.php
index 274d077..30c0e82 100644
--- a/src/Exception/CurrencyConversionException.php
+++ b/src/Exception/CurrencyConversionException.php
@@ -41,7 +41,7 @@ public function __construct(string $message, string $sourceCurrencyCode, string
      *
      * @return CurrencyConversionException
      */
-    public static function exchangeRateNotAvailable(string $sourceCurrencyCode, string $targetCurrencyCode, string $info = null) : self
+    public static function exchangeRateNotAvailable(string $sourceCurrencyCode, string $targetCurrencyCode, ?string $info = null) : self
     {
         $message = sprintf(
             'No exchange rate available to convert %s to %s',
diff --git a/src/Money.php b/src/Money.php
index 9c05b77..dab608b 100644
--- a/src/Money.php
+++ b/src/Money.php
@@ -177,7 +177,7 @@ public static function create(BigNumber $amount, Currency $currency, Context $co
      * @throws NumberFormatException      If the amount is a string in a non-supported format.
      * @throws RoundingNecessaryException If the rounding was necessary to represent the amount at the requested scale.
      */
-    public static function of($amount, $currency, Context $context = null, int $roundingMode = RoundingMode::UNNECESSARY) : Money
+    public static function of($amount, $currency, ?Context $context = null, int $roundingMode = RoundingMode::UNNECESSARY) : Money
     {
         if (! $currency instanceof Currency) {
             $currency = Currency::of($currency);
@@ -209,7 +209,7 @@ public static function of($amount, $currency, Context $context = null, int $roun
      * @throws UnknownCurrencyException If the currency is an unknown currency code.
      * @throws MathException            If the amount cannot be converted to a BigInteger.
      */
-    public static function ofMinor($minorAmount, $currency, Context $context = null, int $roundingMode = RoundingMode::UNNECESSARY) : Money
+    public static function ofMinor($minorAmount, $currency, ?Context $context = null, int $roundingMode = RoundingMode::UNNECESSARY) : Money
     {
         if (! $currency instanceof Currency) {
             $currency = Currency::of($currency);
@@ -235,7 +235,7 @@ public static function ofMinor($minorAmount, $currency, Context $context = null,
      *
      * @return Money
      */
-    public static function zero($currency, Context $context = null) : Money
+    public static function zero($currency, ?Context $context = null) : Money
     {
         if (! $currency instanceof Currency) {
             $currency = Currency::of($currency);
@@ -613,7 +613,7 @@ public function negated() : Money
      * @throws UnknownCurrencyException If an unknown currency code is given.
      * @throws MathException            If the exchange rate or rounding mode is invalid, or rounding is necessary.
      */
-    public function convertedTo($currency, $exchangeRate, Context $context = null, int $roundingMode = RoundingMode::UNNECESSARY) : Money
+    public function convertedTo($currency, $exchangeRate, ?Context $context = null, int $roundingMode = RoundingMode::UNNECESSARY) : Money
     {
         if (! $currency instanceof Currency) {
             $currency = Currency::of($currency);
diff --git a/tests/AbstractTestCase.php b/tests/AbstractTestCase.php
index f72b23a..2697380 100644
--- a/tests/AbstractTestCase.php
+++ b/tests/AbstractTestCase.php
@@ -52,7 +52,7 @@ final protected function assertMoneyEquals(string $expectedAmount, string $expec
      *
      * @return void
      */
-    final protected function assertMoneyIs(string $expected, Money $actual, Context $context = null) : void
+    final protected function assertMoneyIs(string $expected, Money $actual, ?Context $context = null) : void
     {
         $this->assertSame($expected, (string) $actual);