diff --git a/src/Attribute.php b/src/Attribute.php
index ebd7a11..279eb8c 100644
--- a/src/Attribute.php
+++ b/src/Attribute.php
@@ -211,7 +211,7 @@ public function getKeyIndexes(): array
      * @param string|null $key
      * @return mixed
      */
-    public function getValue(string $key = null)
+    public function getValue(?string $key = null)
     {
         if ($key && $this->isArrayAttribute()) {
             $key = $this->resolveSiblingKey($key);
diff --git a/src/Helper.php b/src/Helper.php
index 06e0ada..e47e4cd 100644
--- a/src/Helper.php
+++ b/src/Helper.php
@@ -219,7 +219,7 @@ public static function snakeCase(string $value, string $delimiter = '_'): string
      * @param  string|null  $lastSeparator
      * @return string
      */
-    public static function join(array $pieces, string $separator, string $lastSeparator = null): string
+    public static function join(array $pieces, string $separator, ?string $lastSeparator = null): string
     {
         if (is_null($lastSeparator)) {
             $lastSeparator = $separator;
@@ -245,7 +245,7 @@ public static function join(array $pieces, string $separator, string $lastSepara
      * @param  string|null  $suffix
      * @return array
      */
-    public static function wraps(array $strings, string $prefix, string $suffix = null): array
+    public static function wraps(array $strings, string $prefix, ?string $suffix = null): array
     {
         if (is_null($suffix)) {
             $suffix = $prefix;