ensure parentElement exists

lib/single-file/single-file-helper.js

@@ -198,8 +198,10 @@ this.singlefile.lib.helper = this.singlefile.lib.helper || (() => {
 			getElementsInfo(win, doc, element, options, data, elementHidden);
 			if (!options.autoSaveExternalSave && options.removeHiddenElements && ascendantHidden) {
 				if (elementKept || element.getAttribute(KEPT_CONTENT_ATTRIBUTE_NAME) == "") {
-					element.parentElement.setAttribute(KEPT_CONTENT_ATTRIBUTE_NAME, "");
-					data.markedElements.push(element);
+					if (element.parentElement) {
+						element.parentElement.setAttribute(KEPT_CONTENT_ATTRIBUTE_NAME, "");
+						data.markedElements.push(element.parentElement);
+					}
 				} else if (elementHidden) {
 					element.setAttribute(REMOVED_CONTENT_ATTRIBUTE_NAME, "");
 					data.markedElements.push(element);