diff --git a/src/Compat.php b/src/Compat.php
index 6c020a98..5bbf2a35 100644
--- a/src/Compat.php
+++ b/src/Compat.php
@@ -1863,7 +1863,7 @@ public static function crypto_pwhash(
         string $salt,
         int $opslimit,
         int $memlimit,
-        int $alg = null
+        ?int $alg = null
     ): string {
         if (self::useNewSodiumAPI()) {
             if (!is_null($alg)) {