Почему возникает ошибка: Fatal error: Uncaught Error: Call to a member function item() on null?

$allowedAttributes = ['src', 'href'];
foreach($elements as $element) {
	if( $element->attributes ) {
		for($i = $element->attributes->length; --$i >= 0;) {
			if(!in_array($element->attributes->item($i)->name, $allowedAttributes)) {
				$element->removeAttribute($element->attributes->item($i)->name);
			}
		}
	}
}
echo $dom->saveHTML();