diff --git a/otherlibs/stdune/src/list.ml b/otherlibs/stdune/src/list.ml
index 93fa131738b..7ce0ea75ebc 100644
--- a/otherlibs/stdune/src/list.ml
+++ b/otherlibs/stdune/src/list.ml
@@ -273,3 +273,11 @@ let rec partition_three xs ~f =
      | `Middle y -> xs, y :: ys, zs
      | `Right z -> xs, ys, z :: zs)
 ;;
+
+module Assoc = struct
+  let rec find_exn xs x ~equal =
+    match xs with
+    | [] -> raise Not_found
+    | (x', y) :: xs -> if equal x x' then y else find_exn xs x ~equal
+  ;;
+end
diff --git a/otherlibs/stdune/src/list.mli b/otherlibs/stdune/src/list.mli
index 238b44063c6..3473b9fed8d 100644
--- a/otherlibs/stdune/src/list.mli
+++ b/otherlibs/stdune/src/list.mli
@@ -74,3 +74,7 @@ val to_seq : 'a t -> 'a Seq.t
 (** [list_intersperse t ~sep] returns [t] with [sep] inserted between each pair
     of consecutive values. *)
 val intersperse : 'a t -> sep:'a -> 'a t
+
+module Assoc : sig
+  val find_exn : ('a * 'b) list -> 'a -> equal:('a -> 'a -> bool) -> 'b
+end
diff --git a/src/dune_sexp/decoder.ml b/src/dune_sexp/decoder.ml
index 84b327dfd16..5439f27f71d 100644
--- a/src/dune_sexp/decoder.ml
+++ b/src/dune_sexp/decoder.ml
@@ -590,9 +590,9 @@ and find_cstr
   : type a. (string * a t) list -> Loc.t -> string -> values context -> values -> a
   =
   fun cstrs loc name ctx values ->
-  match List.assoc cstrs name with
-  | Some t -> result ctx (eval t ctx values)
-  | None ->
+  match List.Assoc.find_exn cstrs name ~equal:String.equal with
+  | t -> result ctx (eval t ctx values)
+  | exception Not_found ->
     User_error.raise
       ~loc
       ~hints:(User_message.did_you_mean name ~candidates:(List.map cstrs ~f:fst))