I have a struct I’m trying to derive called Geo.Point. I don’t own it.

require Protocol
Protocol.derive(Jason.Encoder, Geo.Point, only: [:coordinates])
%Geo.Point{} |> Jason.encode
{:error,
 %Protocol.UndefinedError{
   description: "Jason.Encoder protocol must always be explicitly implemented.\n\nIf you own the struct, you can derive the implementation specifying which fields should be encoded to JSON:\n\n    @derive {Jason.Encoder, only: [....]}\n    defstruct ...\n\nIt is also possible to encode all fields, although this should be used carefully to avoid accidentally leaking private information when new fields are added:\n\n    @derive Jason.Encoder\n    defstruct ...\n\nFinally, if you don't own the struct you want to encode to JSON, you may use Protocol.derive/3 placed outside of any module:\n\n    Protocol.derive(Jason.Encoder, NameOfTheStruct, only: [...])\n    Protocol.derive(Jason.Encoder, NameOfTheStruct)\n",
   protocol: Jason.Encoder,
   value: %Geo.Point{coordinates: {0, 0}, properties: %{}, srid: nil}
 }}

But I can’t even get it to work for my own structs

defmodule X do
  # I tried both with and without this line.
  @derive {Jason.Encoder, only: [:id]}
  defstruct [:id]
end
Protocol.derive(Jason.Encoder, X, only: [:id])
%X{id: 1} |> Jason.encode
{:error,
 %Protocol.UndefinedError{
   description: "Jason.Encoder protocol must always be explicitly implemented.\n\nIf you own the struct, you can derive the implementation specifying which fields should be encoded to JSON:\n\n    @derive {Jason.Encoder, only: [....]}\n    defstruct ...\n\nIt is also possible to encode all fields, although this should be used carefully to avoid accidentally leaking private information when new fields are added:\n\n    @derive Jason.Encoder\n    defstruct ...\n\nFinally, if you don't own the struct you want to encode to JSON, you may use Protocol.derive/3 placed outside of any module:\n\n    Protocol.derive(Jason.Encoder, NameOfTheStruct, only: [...])\n    Protocol.derive(Jason.Encoder, NameOfTheStruct)\n",
   protocol: Jason.Encoder,
   value: %X{id: 1}
 }}

I get this whether I use Protocol.derive/3 in the console, or even if I put @derive {Jason.Encoder, only: [:id]} in the struct itself.

What am I missing here?