diff --git a/src/Util/XML.php b/src/Util/XML.php
index e5fa4009f3..c35f19b8a8 100644
--- a/src/Util/XML.php
+++ b/src/Util/XML.php
@@ -118,7 +118,7 @@ class XML
 						$namespace = null;
 					}
 
-					$element->addAttribute($attr_key, $attr_value, $namespace);
+					$element->addAttribute($attr_key, $attr_value ?? '', $namespace);
 				}
 
 				continue;