diff --git a/src/JsRunnerService.php b/src/JsRunnerService.php
index bd8d8ce..a7dc268 100644
--- a/src/JsRunnerService.php
+++ b/src/JsRunnerService.php
@@ -31,7 +31,7 @@ class JsRunnerService implements JsRunnerServiceInterface {
   /**
    * {@inheritdoc}
    */
-  public function jsExec(string $code, bool $noMute = TRUE, callable $extender = NULL, bool $forceRecompile = FALSE, bool $cacheToFile = TRUE): mixed {
+  public function jsExec(string $code, bool $noMute = TRUE, ?callable $extender = NULL, bool $forceRecompile = FALSE, bool $cacheToFile = TRUE): mixed {
     if (!is_dir('../tmp')) {
       mkdir('../tmp');
     }
diff --git a/src/JsRunnerServiceInterface.php b/src/JsRunnerServiceInterface.php
index 4daa1bf..443d809 100644
--- a/src/JsRunnerServiceInterface.php
+++ b/src/JsRunnerServiceInterface.php
@@ -26,6 +26,6 @@ interface JsRunnerServiceInterface {
    * @return mixed
    *   Javascript execution result.
    */
-  public function jsExec(string $code, bool $noMute = TRUE, callable $extender = NULL, bool $forceRecompile = FALSE, bool $cacheToFile = TRUE): mixed;
+  public function jsExec(string $code, bool $noMute = TRUE, ?callable $extender = NULL, bool $forceRecompile = FALSE, bool $cacheToFile = TRUE): mixed;
 
 }