diff --git a/inc/container.class.php b/inc/container.class.php
index c3956d33..f9cd4770 100644
--- a/inc/container.class.php
+++ b/inc/container.class.php
@@ -1145,7 +1145,7 @@ public static function showFormSubtype($params, $display = false)
                     // For delete <sup class='tab_nb'>number</sup> :
                     foreach ($tabs as &$value) {
                         $results = [];
-                        if (preg_match_all('#<sup.*>(.+)</sup>#', $value, $results)) {
+                        if (preg_match_all('#<sup.*>(.+)</sup>#', (string) $value, $results)) {
                             $value = str_replace($results[0][0], '', $value);
                         }
                     }