From f40a911a0ae6ba9db2faaf8372e229f80998e151 Mon Sep 17 00:00:00 2001
From: Josef Moravec <josef.moravec@gmail.com>
Date: Tue, 9 Jan 2024 14:24:22 +0100
Subject: [PATCH] Fix error in items sorting when serial isssue number is null

---
 module/VuFind/src/VuFind/ILS/Driver/KohaRest.php | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

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) {