diff --git a/src/Exportable.php b/src/Exportable.php
index 7aada0f..538e0a0 100644
--- a/src/Exportable.php
+++ b/src/Exportable.php
@@ -56,7 +56,7 @@ public function setColumnStyles($styles): static
      *
      * @return string
      */
-    public function export($path, callable $callback = null)
+    public function export($path, ?callable $callback = null)
     {
         self::exportOrDownload($path, 'openToFile', $callback);
 
@@ -74,7 +74,7 @@ public function export($path, callable $callback = null)
      *
      * @return \Symfony\Component\HttpFoundation\StreamedResponse|string
      */
-    public function download($path, callable $callback = null)
+    public function download($path, ?callable $callback = null)
     {
         if (method_exists(response(), 'streamDownload')) {
             return response()->streamDownload(function () use ($path, $callback) {
@@ -97,7 +97,7 @@ public function download($path, callable $callback = null)
      * @throws \OpenSpout\Writer\Exception\WriterNotOpenedException
      * @throws \OpenSpout\Common\Exception\SpoutException
      */
-    private function exportOrDownload($path, $function, callable $callback = null)
+    private function exportOrDownload($path, $function, ?callable $callback = null)
     {
         if (Str::endsWith($path, 'csv')) {
             $options = new \OpenSpout\Writer\CSV\Options();
diff --git a/src/FastExcel.php b/src/FastExcel.php
index d350ebb..e798b06 100644
--- a/src/FastExcel.php
+++ b/src/FastExcel.php
@@ -61,7 +61,7 @@ class FastExcel
      *
      * @param array|Generator|Collection|null $data
      */
-    public function __construct(array|Generator|Collection $data = null)
+    public function __construct(array|Generator|Collection|null $data = null)
     {
         $this->data = $data;
     }
diff --git a/src/Importable.php b/src/Importable.php
index ae5612d..ad81467 100644
--- a/src/Importable.php
+++ b/src/Importable.php
@@ -39,7 +39,7 @@ abstract protected function setOptions(&$options);
      *
      * @return Collection
      */
-    public function import($path, callable $callback = null)
+    public function import($path, ?callable $callback = null)
     {
         $reader = $this->reader($path);
 
@@ -64,7 +64,7 @@ public function import($path, callable $callback = null)
      *
      * @return Collection
      */
-    public function importSheets($path, callable $callback = null)
+    public function importSheets($path, ?callable $callback = null)
     {
         $reader = $this->reader($path);
 
@@ -142,7 +142,7 @@ private function transposeCollection(array $array)
      *
      * @return array
      */
-    private function importSheet(SheetInterface $sheet, callable $callback = null)
+    private function importSheet(SheetInterface $sheet, ?callable $callback = null)
     {
         $headers = [];
         $collection = [];