diff --git a/src/functions.php b/src/functions.php
index 373afe4..b060fd8 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -9,7 +9,7 @@
  * @param LoopInterface|null $loop
  * @return \React\Promise\PromiseInterface<\Ratchet\Client\WebSocket>
  */
-function connect($url, array $subProtocols = [], $headers = [], LoopInterface $loop = null) {
+function connect($url, array $subProtocols = [], $headers = [], ?LoopInterface $loop = null) {
     $connector = new Connector($loop);
     $connection = $connector($url, $subProtocols, $headers);