diff --git a/src/Formatter/MoneyFormatter.php b/src/Formatter/MoneyFormatter.php
index 1651c9c0..fcd94d51 100644
--- a/src/Formatter/MoneyFormatter.php
+++ b/src/Formatter/MoneyFormatter.php
@@ -107,7 +107,7 @@ public function getCurrency(Money $money): Currency
         return $money->getCurrency();
     }
 
-    protected function getDefaultNumberFormatter(string $currencyCode, string $locale = null): \NumberFormatter
+    protected function getDefaultNumberFormatter(string $currencyCode, ?string $locale = null): \NumberFormatter
     {
         if (is_null($locale)) {
             $locale = \Locale::getDefault();