diff --git a/webservice/soap/classes/class.ilObjectXMLParser.php b/webservice/soap/classes/class.ilObjectXMLParser.php
index 929a2d85c1d2..97de8c3e60d1 100644
--- a/webservice/soap/classes/class.ilObjectXMLParser.php
+++ b/webservice/soap/classes/class.ilObjectXMLParser.php
@@ -58,7 +58,7 @@ public function handlerBeginTag($a_xml_parser, string $a_name, array $a_attribs)
                 $this->addProperty(
                     'obj_id',
                     is_numeric($a_attribs['obj_id']) ? (int) $a_attribs["obj_id"] : ilUtil::__extractId(
-                        $a_attribs["obj_id"],
+                        $a_attribs["obj_id"] ?? '',
                         IL_INST_ID
                     )
                 );