type errors in handwritten conversion function result in weird error

[EmileTrotignon]

type mytype = Int64.t

let mytype_of_json = function
  | `Float f -> Int64.of_float f
  | json -> raise (Melange_json.of_json_error ~json "expected a float")

let mytype_to_json i = Int64.to_string i

type my_record = {
  a : string;
  my_value : mytype;
  b : string;
}
[@@deriving json]

gives the error :

Error: This expression has type (string * [> `String of string ]) list
       but an expression was expected of type (string * string) list
       Type [> `String of string ] is not compatible with type string 

located on the whole of the my_record type definition.

This is quite confusing when the actual error is that mytype_to_json does not return a json but a string. I don't know if something can be done about the location of the error, but at a type constraint could be put on the usage of mytype_to_json so that is error is more like expected Melange_json.t, got string instead of mentioning a tuple and a list.