diff --git a/php/utils/utils.php b/php/utils/utils.php
index 1c5d40e..e39fb77 100644
--- a/php/utils/utils.php
+++ b/php/utils/utils.php
@@ -152,7 +152,7 @@
 	 * Convert special characters to html entities.
 	 */
 	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 == "") {