diff --git a/module/VuFind/src/VuFind/ILS/Driver/KohaRest.php b/module/VuFind/src/VuFind/ILS/Driver/KohaRest.php
index e925b0e72d8..8b4ee9d08f4 100644
--- a/module/VuFind/src/VuFind/ILS/Driver/KohaRest.php
+++ b/module/VuFind/src/VuFind/ILS/Driver/KohaRest.php
@@ -2142,7 +2142,7 @@ protected function statusSortFunction($a, $b)
         $result = $this->getSorter()->compare($a['location'], $b['location']);
 
         if (0 === $result && $this->sortItemsBySerialIssue) {
-            $result = strnatcmp($a['number'], $b['number']);
+            $result = strnatcmp($a['number'] ?? '', $b['number'] ?? '');
         }
 
         if (0 === $result) {