diff --git a/cohttp-eio/src/header.ml b/cohttp-eio/src/header.ml
index 4916181d0..de1218a20 100644
--- a/cohttp-eio/src/header.ml
+++ b/cohttp-eio/src/header.ml
@@ -221,7 +221,7 @@ let make_header_t : #header_definition -> 'a header_t =
 
 type v = V : 'a header * 'a Lazy.t -> v (* Header values are stored lazily. *)
 type binding = B : 'a header * 'a -> binding
-type mapper = { f : 'a. 'a header -> 'a -> 'a }
+type mapper = < f : 'a. 'a header -> 'a -> 'a >
 
 module M = Map.Make (String)
 
@@ -265,13 +265,13 @@ let find_opt k t =
 let iter f t =
   M.iter (fun _key v -> match v with V (k, v) -> f @@ B (k, Lazy.force v)) t.m
 
-let map mapper t =
+let map (m : mapper) t =
   let m =
     M.map
       (fun v ->
         match v with
         | V (k, v) ->
-            let v = mapper.f k @@ Lazy.force v in
+            let v = m#f k @@ Lazy.force v in
             V (k, lazy v))
       t.m
   in