diff --git a/libraries/classes/SqlQueryForm.php b/libraries/classes/SqlQueryForm.php
index 630418ad47..4aa9e4cd78 100644
--- a/libraries/classes/SqlQueryForm.php
+++ b/libraries/classes/SqlQueryForm.php
@@ -410,7 +410,7 @@ class SqlQueryForm
         $html .= '<select name="id_bookmark" id="id_bookmark">' . "\n";
         $html .= '<option value="">&nbsp;</option>' . "\n";
         foreach ($bookmark_list as $bookmark) {
-            $html .= '<option value="' . htmlspecialchars($bookmark->getId()) . '"'
+            $html .= '<option value="' . htmlspecialchars((string)$bookmark->getId()) . '"'
                 . ' data-varcount="' . $bookmark->getVariableCount()
                 . '">'
                 . htmlspecialchars($bookmark->getLabel())