--- 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
--- a/mathexpr/mathexpr.ml	2022-03-22 12:36:30.252580025 +0100
+++ b/mathexpr/mathexpr.ml	2022-03-22 12:32:10.594137416 +0100
@@ -2859,7 +2859,7 @@
 					while true do
 						let n = input f strbuf 0 1024 in
 						if n = 0 then raise Exit else
-						Buffer.add_substring resbuf strbuf 0 n
+						Buffer.add_subbytes resbuf strbuf 0 n
 					done;
 					Buffer.contents resbuf
 					with Exit -> Buffer.contents resbuf