diff --git a/inc/change_item.class.php b/inc/change_item.class.php
index f708490..5e59e3b 100644
--- a/inc/change_item.class.php
+++ b/inc/change_item.class.php
@@ -118,14 +118,14 @@ static function pdfForChange(PluginPdfSimplePDF $pdf, Change $change) {
                      $pdf->displayLine(Toolbox::stripTags(sprintf(__('%1$s: %2$s'), $typename, $nb)),
                                        Toolbox::stripTags($name),
                                        Dropdown::getDropdownName("glpi_entities", $data['entity']),
-                                       Toolbox::stripTags($data["serial"]),
-                                       Toolbox::stripTags($data["otherserial"]),$nb);
+                                       Toolbox::stripTags((string)$data["serial"]),
+                                       Toolbox::stripTags((string)$data["otherserial"]),$nb);
                   } else {
                      $pdf->displayLine('',
                                        Toolbox::stripTags($name),
                                        Dropdown::getDropdownName("glpi_entities", $data['entity']),
-                                       Toolbox::stripTags($data["serial"]),
-                                       Toolbox::stripTags($data["otherserial"]),$nb);
+                                       Toolbox::stripTags((string)$data["serial"]),
+                                       Toolbox::stripTags((string)$data["otherserial"]),$nb);
                   }
                   $prem = false;
                }