diff --git a/src/BasePathDetector.php b/src/BasePathDetector.php
index 001d56d..a4f641f 100644
--- a/src/BasePathDetector.php
+++ b/src/BasePathDetector.php
@@ -23,7 +23,7 @@ class BasePathDetector
      * @param array<mixed> $server The SERVER data to use
      * @param string|null $phpSapi The PHP_SAPI value
      */
-    public function __construct(array $server, string $phpSapi = null)
+    public function __construct(array $server, ?string $phpSapi = null)
     {
         $this->server = $server;
         $this->phpSapi = $phpSapi ?? PHP_SAPI;
diff --git a/src/BasePathMiddleware.php b/src/BasePathMiddleware.php
index a76185b..d11a5a9 100644
--- a/src/BasePathMiddleware.php
+++ b/src/BasePathMiddleware.php
@@ -29,7 +29,7 @@ final class BasePathMiddleware implements MiddlewareInterface
      * @param App $app The slim app
      * @param string|null $phpSapi The PHP_SAPI value
      */
-    public function __construct(App $app, string $phpSapi = null)
+    public function __construct(App $app, ?string $phpSapi = null)
     {
         $this->app = $app;
         $this->phpSapi = $phpSapi;