diff --git a/book/command-line-parsing/README.md b/book/command-line-parsing/README.md
index 7e8a409e2..82f2dca96 100644
--- a/book/command-line-parsing/README.md
+++ b/book/command-line-parsing/README.md
@@ -417,6 +417,49 @@ For usage information, run
 [1]
 ```
 
+One downside of the above approach is that we've lost some of the
+benefits of `Filename.arg_type`, in particular, its support for
+autocomplete.  We can preserve this by instead using `Arg_type.map` to
+add the extra check for a regular file, without losing the
+autocompletion support.
+
+```ocaml file=examples/correct/md5_with_better_custom_arg/md5.ml,part=1
+let regular_file =
+  Command.Arg_type.map Filename.arg_type ~f:(fun filename ->
+      match Sys.is_file filename with
+      | `Yes -> filename
+      | `No -> failwith "Not a regular file"
+      | `Unknown -> failwith "Could not determine if this was a regular file")
+```
+
+(BUT! The following shows a result that's different in terms of the
+error message.  We no longer get):
+
+```
+failed to parse FILENAME value "/dev/null"
+```
+
+Why?
+
+
+```sh dir=examples/correct/md5_with_better_custom_arg
+$ dune exec -- ./md5.exe md5.ml
+         ppx md5.pp.ml (exit 127)
+(cd _build/default && .ppx/57ef275c515ec1fe105f6ff0979f5a61/ppx.exe -o md5.pp.ml --impl md5.ml -corrected-suffix .ppx-corrected -diff-cmd - -dump-ast)
+[1]
+$ dune exec -- ./md5.exe /dev/null
+Error parsing command line:
+
+  (Failure "Not a regular file")
+
+For usage information, run
+
+  md5.exe -help
+
+[1]
+```
+
+
 ### Optional and Default Arguments
 
 A more realistic `md5` binary could also read from the standard input if a
diff --git a/book/command-line-parsing/examples/correct/md5_with_better_custom_arg/.rwo-example b/book/command-line-parsing/examples/correct/md5_with_better_custom_arg/.rwo-example
new file mode 100644
index 000000000..d156db9eb
--- /dev/null
+++ b/book/command-line-parsing/examples/correct/md5_with_better_custom_arg/.rwo-example
@@ -0,0 +1 @@
+(packages core)
diff --git a/book/command-line-parsing/examples/correct/md5_with_better_custom_arg/dune b/book/command-line-parsing/examples/correct/md5_with_better_custom_arg/dune
new file mode 100644
index 000000000..19c5052f1
--- /dev/null
+++ b/book/command-line-parsing/examples/correct/md5_with_better_custom_arg/dune
@@ -0,0 +1,4 @@
+(executable
+  (name       md5)
+  (libraries  core)
+  (preprocess (pps ppx_jane)))
diff --git a/book/command-line-parsing/examples/correct/md5_with_better_custom_arg/dune-project b/book/command-line-parsing/examples/correct/md5_with_better_custom_arg/dune-project
new file mode 100644
index 000000000..7c716d575
--- /dev/null
+++ b/book/command-line-parsing/examples/correct/md5_with_better_custom_arg/dune-project
@@ -0,0 +1,2 @@
+(lang dune 2.6)
+(name rwo-example)
diff --git a/book/command-line-parsing/examples/correct/md5_with_better_custom_arg/dune-workspace b/book/command-line-parsing/examples/correct/md5_with_better_custom_arg/dune-workspace
new file mode 100644
index 000000000..e69de29bb
diff --git a/book/command-line-parsing/examples/correct/md5_with_better_custom_arg/md5.ml b/book/command-line-parsing/examples/correct/md5_with_better_custom_arg/md5.ml
new file mode 100644
index 000000000..3f03e64cc
--- /dev/null
+++ b/book/command-line-parsing/examples/correct/md5_with_better_custom_arg/md5.ml
@@ -0,0 +1,24 @@
+open Core
+
+let do_hash file =
+  Md5.digest_file_blocking file
+  |> Md5.to_hex
+  |> print_endline
+
+[@@@part "1"];;
+let regular_file =
+  Command.Arg_type.map Filename.arg_type ~f:(fun filename ->
+      match Sys.is_file filename with
+      | `Yes -> filename
+      | `No -> failwith "Not a regular file"
+      | `Unknown -> failwith "Could not determine if this was a regular file")
+
+[@@@part "2"];;
+let command =
+  Command.basic ~summary:"Generate an MD5 hash of the input data"
+    ~readme:(fun () -> "More detailed information")
+    Command.Let_syntax.(
+      let%map_open filename = anon ("filename" %: regular_file) in
+      fun () -> do_hash filename)
+
+let () = Command.run ~version:"1.0" ~build_info:"RWO" command
diff --git a/book/command-line-parsing/examples/dune.inc b/book/command-line-parsing/examples/dune.inc
index 2f8fdba25..d3a474190 100644
--- a/book/command-line-parsing/examples/dune.inc
+++ b/book/command-line-parsing/examples/dune.inc
@@ -132,6 +132,18 @@
  (name runtest)
  (deps (alias md5_succinct)))
 
+(rule
+ (alias md5_with_better_custom_arg)
+ (deps
+  (source_tree ./correct/md5_with_better_custom_arg)
+  (package core))
+ (action
+  (system "dune build @all @runtest --root ./correct/md5_with_better_custom_arg")))
+
+(alias
+ (name runtest)
+ (deps (alias md5_with_better_custom_arg)))
+
 (rule
  (alias md5_with_custom_arg)
  (deps