diff --git a/src/Config.php b/src/Config.php
index f46508e..f4f4b05 100644
--- a/src/Config.php
+++ b/src/Config.php
@@ -30,7 +30,7 @@ class Config implements ConfigInterface, ConfigInterpolatorInterface
      * @param array $data
      *   Config data to store.
      */
-    public function __construct(array $data = null)
+    public function __construct(?array $data = null)
     {
         $this->config = new Data($data ?: []);
         $this->setDefaults(new Data());