diff --git a/htdocs/core/lib/functions.lib.php b/htdocs/core/lib/functions.lib.php
index 5c57d266301..f13707b4a36 100644
--- a/htdocs/core/lib/functions.lib.php
+++ b/htdocs/core/lib/functions.lib.php
@@ -7630,9 +7630,14 @@ function dol_string_onlythesehtmlattributes($stringtoclean, $allowed_attributes
 		$dom = new DOMDocument(null, 'UTF-8');
 		$dom->loadHTML($stringtoclean, LIBXML_ERR_NONE | LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD | LIBXML_NONET | LIBXML_NOWARNING | LIBXML_NOXMLDECL);
 
-		if (is_object($dom)) {
+		if ($dom instanceof DOMDocument) {
 			for ($els = $dom->getElementsByTagname('*'), $i = $els->length - 1; $i >= 0; $i--) {
-				for ($attrs = $els->item($i)->attributes, $ii = $attrs->length - 1; $ii >= 0; $ii--) {
+				$el = $els->item($i);
+				if (!$el instanceof DOMElement) {
+					continue;
+				}
+				$attrs = $el->attributes;
+				for ($ii = $attrs->length - 1; $ii >= 0; $ii--) {
 					//var_dump($attrs->item($ii));
 					if (!empty($attrs->item($ii)->name)) {
 						if (! in_array($attrs->item($ii)->name, $allowed_attributes)) {