diff --git a/common/mlstdutils/std_utils.ml b/common/mlstdutils/std_utils.ml
index 70156d1ba..4237ea519 100644
--- a/common/mlstdutils/std_utils.ml
+++ b/common/mlstdutils/std_utils.ml
@@ -269,6 +269,8 @@ module String = struct
         else loop (i+1)
       in
       loop 0
+
+    let unix2dos str = replace str "\n" "\r\n"
 end
 
 module List = struct
diff --git a/common/mlstdutils/std_utils.mli b/common/mlstdutils/std_utils.mli
index 5e5242f7a..3c78f02bc 100644
--- a/common/mlstdutils/std_utils.mli
+++ b/common/mlstdutils/std_utils.mli
@@ -131,6 +131,11 @@ module String : sig
         segment of [str] which contains only bytes {!i not} in [reject].
 
         These work exactly like the C functions [strspn] and [strcspn]. *)
+    val unix2dos : string -> string
+    (** Convert string with ordinary Unix-style line-endings to
+        CRLF DOS-style line-endings.
+
+        The same as {!String.replace} [str "\n" "\r\n"]. *)
 end
 (** Override the String module from stdlib. *)
 
diff --git a/customize/firstboot.ml b/customize/firstboot.ml
index 7adea37f4..a8f4ef7c3 100644
--- a/customize/firstboot.ml
+++ b/customize/firstboot.ml
@@ -24,9 +24,6 @@ open Common_gettext.Gettext
 
 open Regedit
 
-let unix2dos s =
-  String.concat "\r\n" (String.nsplit "\n" s)
-
 let sanitize_name =
   let rex = PCRE.compile ~caseless:true "[^a-z0-9_]" in
   fun n ->
@@ -313,7 +310,8 @@ echo uninstalling firstboot service
 %s -s firstboot uninstall
 " firstboot_dir_win srvany in
 
-    g#write (firstboot_dir // "firstboot.bat") (unix2dos firstboot_script);
+    g#write (firstboot_dir // "firstboot.bat")
+            (String.unix2dos firstboot_script);
 
     (* Open the SYSTEM hive. *)
     Registry.with_hive_write g (g#inspect_get_windows_system_hive root)
@@ -363,7 +361,7 @@ let add_firstboot_script (g : Guestfs.guestfs) root name content =
   | "windows", _ ->
     let firstboot_dir = Windows.install_service g root in
     let filename = firstboot_dir // "scripts" // filename ^ ".bat" in
-    g#write filename (unix2dos content)
+    g#write filename (String.unix2dos content)
 
   | _ ->
     error (f_"guest type %s/%s is not supported") typ distro