diff --git a/lib/meine_seminare_func.inc.php b/lib/meine_seminare_func.inc.php
index 28cb23b8726db7de49eecc824aae00489b1497e9..297bf3fc71b101736b88ce364404d77312eb699f 100644
--- a/lib/meine_seminare_func.inc.php
+++ b/lib/meine_seminare_func.inc.php
@@ -129,7 +129,7 @@ function sort_groups($group_field, &$groups)
                 return (int)($a['gruppe'] - $b['gruppe']);
             } else {
                 if (Config::get()->IMPORTANT_SEMNUMBER) {
-                    return strnatcasecmp($a['sem_nr'], $b['sem_nr']);
+                    return strnatcasecmp($a['sem_nr'] ?? '', $b['sem_nr'] ?? '');
                 } else {
                     return strnatcmp($a['name'], $b['name']);
                 }