diff --git a/src/dune_util/value.ml b/src/dune_util/value.ml
index 4c14223af5e1..4640125ee14a 100644
--- a/src/dune_util/value.ml
+++ b/src/dune_util/value.ml
@@ -42,7 +42,12 @@ let to_path ?error_loc t ~dir =
 let to_path_in_build_or_external ?error_loc t ~dir =
   match t with
   | String s -> Path.relative_to_source_in_build_or_external ?error_loc ~dir s
-  | Dir p | Path p -> p
+  | Dir p | Path p ->
+    if Path.is_in_source_tree p then
+      Code_error.raise ?loc:error_loc
+        "to_path_in_build_or_external got a file in source directory"
+        [ ("path", Path.to_dyn p) ];
+    p
 
 module L = struct
   let to_dyn t = Dyn.List (List.map t ~f:to_dyn)