diff --git a/src/stdune/path.ml b/src/stdune/path.ml
index e5066859..d19410c3 100644
--- a/src/stdune/path.ml
+++ b/src/stdune/path.ml
@@ -667,13 +667,14 @@ let append_local a b =
   | External a -> external_ (External.relative a (Local.to_string b))
 
 let append a b =
-  match kind b with
-  | External _ ->
-    Exn.code_error "Path.append called with non-local second path"
+  match b with
+  | In_build_dir _ | External _ ->
+    Exn.code_error "Path.append called with directory that's \
+                    not in the source tree"
       [ "a", sexp_of_t a
       ; "b", sexp_of_t b
       ]
-  | Local b -> append_local a b
+  | In_source_tree b -> append_local a b
 
 let basename t =
   match kind t with
diff --git a/test/unit-tests/path.mlt b/test/unit-tests/path.mlt
index a956d7b2..0f47a2b8 100644
--- a/test/unit-tests/path.mlt
+++ b/test/unit-tests/path.mlt
@@ -243,12 +243,24 @@ Path.append Path.build_dir (Path.relative Path.root "foo")
 
 Path.append Path.build_dir (Path.relative Path.build_dir "foo")
 [%%expect{|
-- : Path.t = (In_build_dir "_build/foo")
+Exception:
+Code_error
+ (List
+   [Quoted_string
+     "Path.append called with directory that's not in the source tree";
+    List [Atom (A "a"); List [Atom (A "In_build_dir"); Atom (A ".")]];
+    List [Atom (A "b"); List [Atom (A "In_build_dir"); Atom (A "foo")]]]).
 |}]
 
 Path.append Path.root (Path.relative Path.build_dir "foo")
 [%%expect{|
-- : Path.t = (In_source_tree "_build/foo")
+Exception:
+Code_error
+ (List
+   [Quoted_string
+     "Path.append called with directory that's not in the source tree";
+    List [Atom (A "a"); List [Atom (A "In_source_tree"); Atom (A ".")]];
+    List [Atom (A "b"); List [Atom (A "In_build_dir"); Atom (A "foo")]]]).
 |}]
 
 Path.append Path.root (Path.relative Path.root "foo")
@@ -263,7 +275,13 @@ Path.append (Path.of_string "/root") (Path.relative Path.root "foo")
 
 Path.append (Path.of_string "/root") (Path.relative Path.build_dir "foo")
 [%%expect{|
-- : Path.t = (External "/root/_build/foo")
+Exception:
+Code_error
+ (List
+   [Quoted_string
+     "Path.append called with directory that's not in the source tree";
+    List [Atom (A "a"); List [Atom (A "External"); Atom (A "/root")]];
+    List [Atom (A "b"); List [Atom (A "In_build_dir"); Atom (A "foo")]]]).
 |}]
 
 Path.rm_rf (Path.of_string "/does/not/exist/foo/bar/baz")