diff --git a/application/controllers/SelfServiceController.php b/application/controllers/SelfServiceController.php
index d6ac80f3..0b3b642f 100644
--- a/application/controllers/SelfServiceController.php
+++ b/application/controllers/SelfServiceController.php
@@ -275,7 +275,7 @@ class SelfServiceController extends ActionController
             // PluginsUrl => framework_plugins_url
         ];
         $username = $settings->get('self-service/icinga_service_user');
-        if (strlen($username)) {
+        if ($username !== null && strlen($username) > 0) {
             $params['icinga_service_user'] = $username;
         }
 
diff --git a/doc/82-Changelog.md b/doc/82-Changelog.md
index a6015798..d767b4c1 100644
--- a/doc/82-Changelog.md
+++ b/doc/82-Changelog.md
@@ -21,6 +21,9 @@ v1.10.2 (unreleased)
 * FEATURE: more details shown in error messages related to invalid characters (#2646)
 * FIX: snapshots for Baskets containing Baskets failed since v1.10 (#2644)
 
+### REST API
+* FIX: Self Service API returned invalid JSON on PHP 8.1 (#2614)
+
 ### Internals
 * FIX: issue with empty activity log, deprecate outdated method (#2630)