--- ocamlsdl-0.9.1.old/src/sdlmouse.ml	2011-04-10 16:33:52.000000000 +0100
+++ ocamlsdl-0.9.1/src/sdlmouse.ml	2017-11-18 11:24:01.989795950 +0000
@@ -49,12 +49,12 @@
     = "ml_SDL_Cursor_data"
 
 let string_of_bits x =
-  let s = String.make 8 ' ' in
+  let s = Bytes.make 8 ' ' in
   for i=0 to 7 do
     if x land (1 lsl i) <> 0
-    then s.[7-i] <- '@'
+    then Bytes.set s (7-i) '@'
   done ;
-  s
+  Bytes.to_string s
 
 let pprint_cursor c =
   let { data = data ; mask = mask } = cursor_data c in