diff --git a/LibreNMS/Alert/Transport/Sensu.php b/LibreNMS/Alert/Transport/Sensu.php
index a6b0b4db82..b39222424f 100644
--- a/LibreNMS/Alert/Transport/Sensu.php
+++ b/LibreNMS/Alert/Transport/Sensu.php
@@ -151,7 +151,7 @@ class Sensu extends Transport
             'librenms-device-id' => strval($obj['device_id']),
             'librenms-rule-id' => strval($obj['rule_id']),
             'librenms-status-reason' => $obj['status_reason'],
-        ], function (string $s): bool {
+        ], function (?string $s): bool {
             return (bool) strlen($s); // strlen returns 0 for null, false or '', but 1 for integer 0 - unlike empty()
         });
     }