diff --git a/api/ParserService.js b/api/ParserService.js
index fa3c570c87..19be682658 100644
--- a/api/ParserService.js
+++ b/api/ParserService.js
@@ -264,6 +264,9 @@ var roundTripDiff = function ( req, res, src, document ) {
 	res.write(document.body.innerHTML + '<hr>');
 	res.write( '<h2>HTML DOM converted back to Wikitext</h2><hr>' );
 	var out = new WikitextSerializer({env: env}).serializeDOM( document.body );
+	if ( out === undefined ) {
+		out = "An error occured in the WikitextSerializer, please check the log for information";
+	}
 	res.write('<pre>' + htmlSpecialChars( out ) + '</pre><hr>\n');
 	res.write( '<h2>Diff between original Wikitext (green) and round-tripped wikitext (red)</h2><hr>\n' );
 	var patch;