diff --git spamoracle/mail.ml spamoracle/mail.ml
index 92d46e7..742771e 100644
--- spamoracle/mail.ml
+++ spamoracle/mail.ml
@@ -188,7 +188,7 @@ let run_body_through_external_converter cmd arg body =
     None
   end else begin
     let len = in_channel_length ic in
-    let res = String.create len in
+    let res = Bytes.create len in
     really_input ic res 0 len;
     close_in ic;
     safe_remove infile; safe_remove outfile;