diff --git a/src/CaptchaBuilder.php b/src/CaptchaBuilder.php
index 5d633be..6edda58 100644
--- a/src/CaptchaBuilder.php
+++ b/src/CaptchaBuilder.php
@@ -140,7 +140,7 @@ public function setInterpolation($interpolate = true)
      */
     public $tempDir = 'temp/';
 
-    public function __construct($phrase = null, PhraseBuilderInterface $builder = null)
+    public function __construct($phrase = null, ?PhraseBuilderInterface $builder = null)
     {
         if ($builder === null) {
             $this->builder = new PhraseBuilder;