diff --git a/atdgen/src/ocaml.ml b/atdgen/src/ocaml.ml
index d9f604f0..4cf01ccc 100644
--- a/atdgen/src/ocaml.ml
+++ b/atdgen/src/ocaml.ml
@@ -661,6 +661,7 @@ let rec ocaml_of_expr_mapping (x : (Repr.t, _) mapping) : ocaml_expr =
       `Name (s, List.map ocaml_of_expr_mapping l)
   | Tvar (_, s) ->
       `Tvar s
+  | Abstract _ -> `Name ("Yojson.Safe.t", [])
   | _ -> assert false
 
 and ocaml_of_variant_mapping x =
diff --git a/atdgen/src/ov_emit.ml b/atdgen/src/ov_emit.ml
index 554624da..68374e74 100644
--- a/atdgen/src/ov_emit.ml
+++ b/atdgen/src/ov_emit.ml
@@ -149,7 +149,8 @@ let rec get_validator_name
   | Bool (_, Bool, v)
   | Int (_, Int _, v)
   | Float (_, Float, v)
-  | String (_, String, v) ->
+  | String (_, String, v)
+  | Abstract (_, Abstract, v) ->
       (match v with
          (None, true) -> return_true_paren
        | (Some s, true) -> s
@@ -202,6 +203,7 @@ let rec make_validator (x : ov_mapping) : Indent.t list =
   | String _
   | Name _
   | External _
+  | Abstract _
   | Tvar _ -> [ Line (get_validator_name x) ]
 
   | Sum (_, a, Sum x, (v, shallow)) ->
diff --git a/atdgen/src/ov_mapping.ml b/atdgen/src/ov_mapping.ml
index a6e9eb9b..198dccf2 100644
--- a/atdgen/src/ov_mapping.ml
+++ b/atdgen/src/ov_mapping.ml
@@ -207,6 +207,8 @@ let rec mapping_of_expr
            Float (loc, Float, (v an, true))
        | "string" ->
            String (loc, String, (v an, true))
+       | "abstract" ->
+           Abstract (loc, Abstract, (v an, true))
        | s ->
            let validator =
              match v2 an x0 with
@@ -307,13 +309,14 @@ let def_of_atd is_shallow (loc, (name, param, an), x) =
   let o =
     match as_abstract x with
     | Some (_, an2) ->
-        Ocaml.get_ocaml_module_and_t Validate name an
-        |> Option.map (fun (types_module, main_module, ext_name) ->
-          let args = List.map (fun s -> Tvar (loc, s)) param in
-          External (loc, name, args,
-                    Ocaml.Repr.External (types_module, main_module, ext_name),
-                    (Validate.get_validator an2, false))
-        )
+        (match Ocaml.get_ocaml_module_and_t Validate name an with
+          | None -> Some (mapping_of_expr is_shallow x)
+          | Some (types_module, main_module, ext_name) ->
+              let args = List.map (fun s -> Tvar (loc, s)) param in
+              Some (External (loc, name, args,
+                              Ocaml.Repr.External (types_module, main_module, ext_name),
+                              (Validate.get_validator an2, false))))
+
     | None -> Some (mapping_of_expr is_shallow x)
   in
   {