diff --git a/php/utils/server-utils.php b/php/utils/server-utils.php
index 7501be2..cb632f3 100644
--- a/php/utils/server-utils.php
+++ b/php/utils/server-utils.php
@@ -120,7 +120,7 @@
 	}
 
 	function html_sanitize(
-		string $str, int $flags = ENT_QUOTES|ENT_SUBSTITUTE,
+		?string $str, int $flags = ENT_QUOTES|ENT_SUBSTITUTE,
 		?string $encoding = null, bool $double_encode = true
 	) {
 		if ($str == "") {