diff --git a/function.php b/function.php
index e6eec30e..51d23756 100644
--- a/function.php
+++ b/function.php
@@ -763,7 +763,7 @@ if (!function_exists('process_name_set')) {
      * @param string|null $prefix
      * @throws
      */
-    function process_name_set(int $pid, string $prefix = NULL): void
+    function process_name_set(int $pid, ?string $prefix = NULL): void
     {
         if (Kiri::getPlatform()->isMac()) {
             return;