Post.parent() gives wrong query.

[VandorpeDavid]

post |> Post.parent() |> Repo.one()

results in

** (Ecto.Query.CastError) deps/ecto_materialized_path/lib/ecto_materialized_path.ex:73: value 4 in where cannot be cast to type {:in, :id} in query:

from a in Post where: a.id in ^4, limit: 1, select: a

(elixir) lib/enum.ex:1899: Enum."-reduce/3-lists^foldl/2-0-"/3
(elixir) lib/enum.ex:1397: Enum."-map_reduce/3-lists^mapfoldl/2-0-"/3
(elixir) lib/enum.ex:1899: Enum."-reduce/3-lists^foldl/2-0-"/3
(ecto) lib/ecto/repo/queryable.ex:124: Ecto.Repo.Queryable.execute/5
(ecto) lib/ecto/repo/queryable.ex:37: Ecto.Repo.Queryable.all/4
(ecto) lib/ecto/repo/queryable.ex:70: Ecto.Repo.Queryable.one/4
(stdlib) erl_eval.erl:677: :erl_eval.do_apply/6
(iex) lib/iex/evaluator.ex:250: IEx.Evaluator.handle_eval/5
(iex) lib/iex/evaluator.ex:230: IEx.Evaluator.do_eval/3
(iex) lib/iex/evaluator.ex:208: IEx.Evaluator.eval/3

Running post |> Post.parent() returns

#Ecto.Query<from a in Post, where: a.id in ^4, limit: 1>

The use of the in operator seems weird to me, but it has been there for over a year, and noone has complained yet. This makes me wonder if I am doing something wrong.