diff --git a/src/odoc/odoc_file.ml b/src/odoc/odoc_file.ml
index 67f9b07098..cc96b68ef2 100644
--- a/src/odoc/odoc_file.ml
+++ b/src/odoc/odoc_file.ml
@@ -29,13 +29,24 @@ type content =
 type t = { content : content; warnings : Odoc_model.Error.t list }
 
 (** Written at the top of the files. Checked when loading. *)
-let magic = "odoc-%%VERSION%%"
+let magic = "ODOC"
+
+let magic_version = "%%VERSION%%"
 
 (** Exceptions while saving are allowed to leak. *)
 let save_ file f =
+  let len = String.length magic_version in
+  (* Sanity check, see similar check in load_ *)
+  if len > 255 then
+    failwith
+      (Printf.sprintf
+         "Magic version string %S is too long, must be <= 255 characters" magic);
+
   Fs.Directory.mkdir_p (Fs.File.dirname file);
   Io_utils.with_open_out_bin (Fs.File.to_string file) (fun oc ->
       output_string oc magic;
+      output_binary_int oc len;
+      output_string oc magic_version;
       f oc)
 
 let save_unit file (root : Root.t) (t : t) =
@@ -78,19 +89,41 @@ let save_unit file ~warnings m =
 
 let load_ file f =
   let file = Fs.File.to_string file in
-  (if Sys.file_exists file then Ok file
-   else Error (`Msg (Printf.sprintf "File does not exist")))
-  >>= fun file ->
-  Io_utils.with_open_in_bin file @@ fun ic ->
-  try
+
+  let check_exists () =
+    if Sys.file_exists file then Ok ()
+    else Error (`Msg (Printf.sprintf "File %s does not exist" file))
+  in
+
+  let check_magic ic =
     let actual_magic = really_input_string ic (String.length magic) in
-    if actual_magic = magic then f ic
+    if actual_magic = magic then Ok ()
+    else
+      Error
+        (`Msg
+           (Printf.sprintf "%s has invalid magic %S, expected %S\n%!" file
+              actual_magic magic))
+  in
+  let version_length ic () =
+    let len = input_binary_int ic in
+    if len > 0 && len <= 255 then Ok len
+    else Error (`Msg (Printf.sprintf "%s has invalid version length" file))
+  in
+  let check_version ic len =
+    let actual_magic = really_input_string ic len in
+    if actual_magic = magic_version then Ok ()
     else
       let msg =
-        Printf.sprintf "%s: invalid magic number %S, expected %S\n%!" file
-          actual_magic magic
+        Printf.sprintf "%s has invalid version %S, expected %S\n%!" file
+          actual_magic magic_version
       in
       Error (`Msg msg)
+  in
+
+  check_exists () >>= fun () ->
+  Io_utils.with_open_in_bin file @@ fun ic ->
+  try
+    check_magic ic >>= version_length ic >>= check_version ic >>= fun () -> f ic
   with exn ->
     let msg =
       Printf.sprintf "Error while unmarshalling %S: %s\n%!" file