diff --git a/src/Google/Ads/GoogleAds/Util/ApiVersionSupport.php b/src/Google/Ads/GoogleAds/Util/ApiVersionSupport.php
index 6b6c6cdbdd..eb2489045c 100644
--- a/src/Google/Ads/GoogleAds/Util/ApiVersionSupport.php
+++ b/src/Google/Ads/GoogleAds/Util/ApiVersionSupport.php
@@ -38,7 +38,7 @@ class ApiVersionSupport
      * @param string|null $rootPath the root path of the library, the one that contains this class
      *     file is used by default
      */
-    public function __construct(string $rootPath = null)
+    public function __construct(?string $rootPath = null)
     {
         $this->rootPath = $rootPath ?: dirname(__DIR__, 5);
     }