diff --git a/ppx/test/classify_drop_option.t b/ppx/test/classify_drop_option.t
new file mode 100644
index 0000000..f5c3309
--- /dev/null
+++ b/ppx/test/classify_drop_option.t
@@ -0,0 +1,81 @@
+We can classify the JSON produced by `to_json` for a record with a dropped optional field:
+  $ echo '
+  > open Melange_json.Primitives
+  > type t = {a: int option; [@json.option] [@json.drop_default]} [@@deriving to_json]
+  > let () =
+  >   match Melange_json.classify (to_json {a = None}) with
+  >   | `Assoc xs -> xs |> List.iter (fun (k, v) ->
+  >       let _ = k in
+  >       let _v = Melange_json.classify v in
+  >       ());
+  >       print_endline "OK"
+  >   | _ -> print_endline "ERROR: Expected an object"
+  > ' | ./run.sh
+  === ppx output:native ===
+  open Melange_json.Primitives
+  type t = {
+    a: int option [@json.option ][@json.drop_default ]}[@@deriving to_json]
+  include
+    struct
+      let _ = fun (_ : t) -> ()
+      [@@@ocaml.warning "-39-11-27"]
+      let rec to_json =
+        (fun x ->
+           match x with
+           | { a = x_a } ->
+               `Assoc
+                 (let bnds__001_ = [] in
+                  let bnds__001_ =
+                    match x_a with
+                    | Stdlib.Option.None -> bnds__001_
+                    | Stdlib.Option.Some _ ->
+                        ("a", ((option_to_json int_to_json) x_a)) :: bnds__001_ in
+                  bnds__001_) : t -> Yojson.Basic.t)
+      let _ = to_json
+    end[@@ocaml.doc "@inline"][@@merlin.hide ]
+  let () =
+    match Melange_json.classify (to_json { a = None }) with
+    | `Assoc xs ->
+        (xs |>
+           (List.iter
+              (fun (k, v) ->
+                 let _ = k in let _v = Melange_json.classify v in ()));
+         print_endline "OK")
+    | _ -> print_endline "ERROR: Expected an object"
+  === ppx output:browser ===
+  open Melange_json.Primitives
+  type t = {
+    a: int option [@json.option ][@json.drop_default ]}[@@deriving to_json]
+  include
+    struct
+      let _ = fun (_ : t) -> ()
+      [@@@ocaml.warning "-39-11-27"]
+      let rec to_json =
+        (fun x ->
+           match x with
+           | { a = x_a } ->
+               (Obj.magic
+                  ([%mel.obj
+                     {
+                       a =
+                         (match x_a with
+                          | Stdlib.Option.None -> Js.Undefined.empty
+                          | Stdlib.Option.Some _ ->
+                              Js.Undefined.return
+                                ((option_to_json int_to_json) x_a))
+                     }]) : Js.Json.t) : t -> Js.Json.t)
+      let _ = to_json
+    end[@@ocaml.doc "@inline"][@@merlin.hide ]
+  let () =
+    match Melange_json.classify (to_json { a = None }) with
+    | `Assoc xs ->
+        (xs |>
+           (List.iter
+              (fun (k, v) ->
+                 let _ = k in let _v = Melange_json.classify v in ()));
+         print_endline "OK")
+    | _ -> print_endline "ERROR: Expected an object"
+  === stdout:native ===
+  OK
+  === stdout:js ===
+  OK
diff --git a/src/classify.ml b/src/classify.ml
index 51c1b6a..6289893 100644
--- a/src/classify.ml
+++ b/src/classify.ml
@@ -2,6 +2,18 @@ module J = Js.Json
 
 type t = J.t
 
+let dict_to_list_without_undefined dict =
+  let keys = Js.Dict.keys dict in
+  let l = Array.length keys in
+  let xs = ref [] in
+  for i = l - 1 downto 0 do
+    let key = Array.unsafe_get keys i in
+    let value = Js.Dict.unsafeGet dict key in
+    if not ((Obj.magic value : _ Js.Undefined.t) == Js.undefined) then
+      xs := (key, value) :: !xs
+  done;
+  !xs
+
 let classify :
     t ->
     [ `Null
@@ -27,8 +39,9 @@ let classify :
           let xs = Array.to_list (Obj.magic json : t array) in
           `List xs
         else
-          let xs = Js.Dict.entries (Obj.magic json : t Js.Dict.t) in
-          `Assoc (Array.to_list xs)
+          `Assoc
+            (dict_to_list_without_undefined
+               (Obj.magic json : t Js.Dict.t))
     | typ -> failwith ("unknown JSON value type: " ^ typ)
 
 let declassify :