--- a/fls/fls.ml	2021-04-26 16:24:25.363442099 +0200
+++ b/fls/fls.ml	2021-04-26 20:10:54.965405870 +0200
@@ -750,7 +750,7 @@
 		match input inp s 0 1024 with
 		0 -> Buffer.contents res
 		| n -> 
-			Buffer.add_substring res s 0 n;
+			Buffer.add_subbytes res s 0 n;
 			fill_buff ()
 	in
 	let s = fill_buff () in