Foreign key error when deleting a list

[Joe Van Dyk]

[debug] HANDLE EVENT "delete-list" in TodoTrekWeb.HomeLive
  Parameters: %{"id" => "4"}
[debug] QUERY OK source="lists" db=8.7ms queue=0.8ms idle=1712.3ms
SELECT l0."id", l0."title", l0."position", l0."user_id", l0."notifications", l0."inserted_at", l0."updated_at" FROM "lists" AS l0 WHERE (l0."user_id" = $1) AND (l0."id" = $2) [1, 4]
↳ TodoTrek.Todos.get_list!/2, at: lib/todo_trek/todos.ex:315
[debug] QUERY OK source="todos" db=0.5ms idle=1722.1ms
SELECT t0."id", t0."status", t0."title", t0."position", t0."list_id", t0."user_id", t0."inserted_at", t0."updated_at", t0."list_id" FROM "todos" AS t0 WHERE (t0."list_id" = $1) ORDER BY t0."list_id" [4]
↳ TodoTrekWeb.HomeLive.handle_event/3, at: lib/todo_trek_web/live/home_live.ex:148
[debug] QUERY OK db=0.1ms idle=1722.7ms
begin []
↳ TodoTrek.Todos.delete_list/2, at: lib/todo_trek/todos.ex:397
[debug] QUERY OK db=0.1ms
SELECT pg_advisory_xact_lock(8174290, 1) []
[debug] QUERY OK source="lists" db=0.6ms
UPDATE "lists" AS l0 SET "position" = l0."position" + -1 WHERE (l0."user_id" = $1) AND (l0."position" > (SELECT sl0."position" FROM "lists" AS sl0 WHERE (sl0."id" = $2))) [1, 4]
↳ TodoTrek.Todos.delete_list/2, at: lib/todo_trek/todos.ex:397
[debug] QUERY ERROR db=0.6ms
DELETE FROM "lists" WHERE "id" = $1 [4]
↳ TodoTrek.Todos.delete_list/2, at: lib/todo_trek/todos.ex:397
[debug] QUERY OK db=0.1ms
rollback []
↳ TodoTrek.Todos.delete_list/2, at: lib/todo_trek/todos.ex:397
[error] GenServer #PID<0.804.0> terminating
** (Ecto.ConstraintError) constraint error when attempting to delete struct:

    * "todos_list_id_fkey" (foreign_key_constraint)

If you would like to stop this constraint violation from raising an
exception and instead add it as an error to your changeset, please
call `foreign_key_constraint/3` on your changeset with the constraint
`:name` as an option.

The changeset has not defined any constraint.