diff --git a/lib/Controller/MementoFinder.php b/lib/Controller/MementoFinder.php
index 4bb53d5..193567d 100644
--- a/lib/Controller/MementoFinder.php
+++ b/lib/Controller/MementoFinder.php
@@ -182,6 +182,7 @@ function extractMementoInfo($file) {
 	$content = $file->getContent();
 	$DOM = new DOMDocument;
 	$DOM->loadHTML($content);
+	if (!$DOM->documentElement) return null; // possibly $content was not HTML at all.
 	$headElement = $DOM->documentElement->getElementsByTagName('head')[0];
 	if (!$headElement) return null; // possibly $content was not HTML at all.
 	$originalUrls = getOriginalUrls($headElement);