diff --git a/src/dir_contents.ml b/src/dir_contents.ml
index ac092002..2debaeb9 100644
--- a/src/dir_contents.ml
+++ b/src/dir_contents.ml
@@ -91,11 +91,12 @@ end = struct
              \nThis will become an error in the future."
             (let tag = Sexp.unsafe_atom_of_string
                          "modules_without_implementation" in
-             Sexp.to_string ~syntax:Dune
-               (List [ tag
-                     ; Sexp.To_sexp.(list string)
-                         (uncapitalized should_be_listed)
-                     ]))
+             let modules =
+               should_be_listed
+               |> uncapitalized
+               |> List.map ~f:Sexp.To_sexp.string
+             in
+             Sexp.to_string ~syntax:Dune (List (tag :: modules)))
         | Some loc ->
           let list_modules l =
             uncapitalized l
diff --git a/test/blackbox-tests/test-cases/intf-only/run.t b/test/blackbox-tests/test-cases/intf-only/run.t
index 3d930d80..f1e40162 100644
--- a/test/blackbox-tests/test-cases/intf-only/run.t
+++ b/test/blackbox-tests/test-cases/intf-only/run.t
@@ -27,7 +27,7 @@ Errors:
   Warning: Some modules don't have an implementation.
   You need to add the following field to this stanza:
   
-    (modules_without_implementation (x y))
+    (modules_without_implementation x y)
   
   This will become an error in the future.
         ocamlc .foo.objs/foo.{cmi,cmo,cmt}