diff --git a/src/Generator.php b/src/Generator.php
index 4ada416..18bd1d3 100644
--- a/src/Generator.php
+++ b/src/Generator.php
@@ -166,7 +166,7 @@ public function __call($method, array $arguments)
      * @throws WriterException
      * @throws InvalidArgumentException
      */
-    public function generate(string $text, string $filename = null)
+    public function generate(string $text, ?string $filename = null)
     {
         $qrCode = $this->getWriter($this->getRenderer())->writeString($text, $this->encoding, $this->errorCorrection);