diff --git a/js/cohttp_lwt_xhr.ml b/js/cohttp_lwt_xhr.ml
index 9e0ad34353..bde2f4e539 100644
--- a/js/cohttp_lwt_xhr.ml
+++ b/js/cohttp_lwt_xhr.ml
@@ -93,8 +93,7 @@ end) = struct
 
   let post_form ?ctx ?headers ~params uri =
     let headers = C.Header.add_opt headers "content-type" "application/x-www-form-urlencoded" in
-    let q = List.map (fun (k,v) -> k, [v]) (C.Header.to_list params) in
-    let body = Cohttp_lwt_body.of_string (Uri.encoded_of_query q) in
+    let body = Cohttp_lwt_body.of_string (Uri.encoded_of_query params) in
     post ?ctx ~chunked:false ~headers ~body uri
 
   (* No implementation (can it be done?).  What should the failure exception be? *)
diff --git a/lwt/cohttp_lwt.ml b/lwt/cohttp_lwt.ml
index 582eb7cfff..ec8c1d8389 100644
--- a/lwt/cohttp_lwt.ml
+++ b/lwt/cohttp_lwt.ml
@@ -105,7 +105,7 @@ module type Client = sig
   val post_form :
     ?ctx:ctx ->
     ?headers:Cohttp.Header.t ->
-    params:Cohttp.Header.t ->
+    params:(string * string list) list ->
     Uri.t -> (Response.t * Cohttp_lwt_body.t) Lwt.t
 
   val callv :
@@ -184,8 +184,7 @@ module Make_client
 
   let post_form ?ctx ?headers ~params uri =
     let headers = Header.add_opt headers "content-type" "application/x-www-form-urlencoded" in
-    let q = List.map (fun (k,v) -> k, [v]) (Header.to_list params) in
-    let body = Cohttp_lwt_body.of_string (Uri.encoded_of_query q) in
+    let body = Cohttp_lwt_body.of_string (Uri.encoded_of_query params) in
     post ?ctx ~chunked:false ~headers ~body uri
 
   let callv ?(ctx=default_ctx) uri reqs =
diff --git a/lwt/cohttp_lwt.mli b/lwt/cohttp_lwt.mli
index b758b2a8ea..688e87d60d 100644
--- a/lwt/cohttp_lwt.mli
+++ b/lwt/cohttp_lwt.mli
@@ -128,7 +128,7 @@ module type Client = sig
   val post_form :
     ?ctx:ctx ->
     ?headers:Cohttp.Header.t ->
-    params:Cohttp.Header.t ->
+    params:(string * string list) list ->
     Uri.t -> (Response.t * Cohttp_lwt_body.t) Lwt.t
 
   val callv :