diff -urN ../roundcubemail-1.6.3.orig/program/lib/Roundcube/rcube_ldap.php ./program/lib/Roundcube/rcube_ldap.php
--- ../roundcubemail-1.6.3.orig/program/lib/Roundcube/rcube_ldap.php	2023-09-15 11:08:28.000000000 +0300
+++ ./program/lib/Roundcube/rcube_ldap.php	2023-09-23 13:53:39.223613000 +0300
@@ -804,6 +804,12 @@
      */
     function _entry_sort_cmp($a, $b)
     {
+        if ((!isset($a[$this->sort_col][0])) or ($a[$this->sort_col][0] === null)) {
+            return(((!isset($b[$this->sort_col][0])) or ($b[$this->sort_col][0] === null)) ? 0 : -1);
+        }
+        if ((!isset($b[$this->sort_col][0])) or ($b[$this->sort_col][0] === null)) {
+            return(((!isset($a[$this->sort_col][0])) or ($a[$this->sort_col][0] === null)) ? 0 : 1);
+        }
         return strcmp($a[$this->sort_col][0] ?? '', $b[$this->sort_col][0] ?? '');
     }