Fix warning on 4.11

Author: andersfugmann

Changelog

@@ -9,6 +9,10 @@ Changes marked with '*' indicates a changes that breaks backward compatibility
 [ ] Add namespaces to attributes
 [ ] Make sure List.map is tail recursive for list types
 
+5.1.3
+[x] Remove dependency on stdio
+[x] Fix warning when compiling with 4.11
+
 5.1.2 (2020-06-26)
 [x] Compatible with Base v0.14
 

runtime/runtime.ml

@@ -253,7 +253,7 @@ module Helper = struct
       The [map_f] function is called to produce the serialized result from a field_name, t association list.
       If [omit_default] is true, then default values are omitted from the output
   *)
-  let of_record: type t a t. omit_default:bool -> t serialize_record -> (t, a, t) Record_out.t -> a =
+  let of_record: type a t. omit_default:bool -> t serialize_record -> (t, a, t) Record_out.t -> a =
     fun ~omit_default serialize_record ->
     let rec inner: type a. (t, a, t) Record_out.t -> (string * t) list -> a =
       let open Record_out in