--- a/scansion/pretty.lua	Thu Sep 13 16:05:24 2018 +0100
+++ b/scansion/pretty.lua	Thu Sep 13 16:05:45 2018 +0100
@@ -66,7 +66,10 @@
 	end
 
 	return function (s, ind)
-		local doc = assert(xml.parse(s));
+		local doc = xml.parse(s);
+		if not doc then
+			return s; -- Not valid XML, don't prettify
+		end
 		return pretty_print(doc, ind);
 	end
 end