diff --git a/src/stdune/path.ml b/src/stdune/path.ml
index 8f58d7d0..96961419 100644
--- a/src/stdune/path.ml
+++ b/src/stdune/path.ml
@@ -468,12 +468,16 @@ let insert_after_build_dir_exn =
       ]
   in
   fun a b ->
-    if not (is_local a) || String.contains b '/' then error a b;
-    match String.lsplit2 a ~on:'/' with
-    | Some ("_build", rest) ->
-      Printf.sprintf "_build/%s/%s" b rest
-    | _ ->
+    if not (is_local a) then
       error a b
+    else if a = build_dir then
+      relative build_dir b
+    else
+      match String.lsplit2 a ~on:'/' with
+      | Some (build_dir', rest) when build_dir = build_dir' ->
+        Local.append (relative build_dir b) rest
+      | _ ->
+        error a b
 
 let rm_rf =
   let rec loop dir =
diff --git a/test/unit-tests/path.mlt b/test/unit-tests/path.mlt
index c31ba466..bffb0366 100644
--- a/test/unit-tests/path.mlt
+++ b/test/unit-tests/path.mlt
@@ -111,3 +111,20 @@ Path.is_local (Path.absolute "relative/path")
 [%%expect{|
 - : bool = false
 |}]
+
+Path.insert_after_build_dir_exn Path.root "foobar"
+[%%expect{|
+Exception: Stdune__Exn.Code_error <abstr>.
+Raised at file "src/stdune/exn.ml", line 30, characters 37-131
+Called from file "toplevel/toploop.ml", line 180, characters 17-56
+|}]
+
+Path.insert_after_build_dir_exn Path.build_dir "foobar"
+[%%expect{|
+- : Stdune.Path.t = _build/foobar
+|}]
+
+Path.insert_after_build_dir_exn (Path.relative Path.build_dir "qux") "foobar"
+[%%expect{|
+- : Stdune.Path.t = _build/foobar/qux
+|}]