diff --git a/src/Image.php b/src/Image.php
index b4a049a..87ea7d2 100644
--- a/src/Image.php
+++ b/src/Image.php
@@ -308,7 +308,7 @@ public function curl(string $url, array $curlOptions = [], bool $failOnError = f
     {
         $defaultCurlOptions = [
             CURLOPT_USERAGENT => 'Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0) Gecko/20100101 Firefox/116.0',
-            CURLOPT_REFERER => \strtolower($_SERVER["REQUEST_SCHEME"] . '://' . $_SERVER["HTTP_HOST"] . $_SERVER["REQUEST_URI"]),
+            CURLOPT_REFERER => \strtolower(($_SERVER["REQUEST_SCHEME"] ?? 'http') . '://' . $_SERVER["HTTP_HOST"] . $_SERVER["REQUEST_URI"]),
             CURLOPT_RETURNTRANSFER => 1,
             CURLOPT_TIMEOUT => 5,
         ];