diff --git a/src/lib/fileutils/FileUtil.mli b/src/lib/fileutils/FileUtil.mli
index 89904aa..8b9f912 100644
--- a/src/lib/fileutils/FileUtil.mli
+++ b/src/lib/fileutils/FileUtil.mli
@@ -498,8 +498,8 @@ val which:
 
 (** [cmp skip1 fln1 skip2 fln2] Compare files [fln1] and [fln2] starting at pos
     [skip1] [skip2] and returning the first octect where a difference occurs.
-    Returns [Some -1] if one of the file is not readable or if it is not a
-    file.
+    Returns [Some -1] if one of the files is not readable or if it is not a
+    file.  Returns [None] if given two identical files.
     See {{:http://pubs.opengroup.org/onlinepubs/007904875/utilities/cmp.html}POSIX documentation}.
   *)
 val cmp:
diff --git a/src/lib/fileutils/FileUtilCMP.ml b/src/lib/fileutils/FileUtilCMP.ml
index 88f7c0c..4e43747 100644
--- a/src/lib/fileutils/FileUtilCMP.ml
+++ b/src/lib/fileutils/FileUtilCMP.ml
@@ -48,11 +48,14 @@ let cmp ?(skip1 = 0) fln1 ?(skip2 = 0) fln2 =
     let rec loop count s1 s2 =
       match s1, s2 with
       | Seq.Cons (v1, s1), Seq.Cons (v2, s2) when v1 = v2 -> loop (count + 1) (s1 ()) (s2 ())
+      | Seq.Nil, Seq.Nil -> (-1)
       | _ -> count
     in
     let count = loop 0 (stream1 ()) (stream2 ()) in
     clean_fd ();
-    Some count
+    match count with
+    | (-1) -> None
+    | x -> Some x
   end else
     Some (-1)