Not_null_violation on seeding my database: timestamps() macro failing?

I’m able to create and migrate my database tables Ok. But when I try to seed it, I’m getting errors as soon as I encounter a table that uses the timestamps() macro in its migration definition:

** (Postgrex.Error) ERROR 23502 (not_null_violation) null value in column "inserted_at" violates not-null constraint

    table: tenants
    column: inserted_at

My migration looks like this:

defmodule Auth.Repo.Migrations.Tenants do
  use Ecto.Migration

  def change do
    create table(:tenants, primary_key: false) do
      add :id, :string, size: 16, null: false, primary_key: true # Slug
      add :status_id, references("tenants_statuses", type: :string), size: 10, null: false
      add :name, :string, size: 32, null: false, default: ""
      add :description, :text, null: false, default: ""
      timestamps()
    end
  end
end

And my seeds.exs like this:

alias Auth.Repo

Repo.insert_all(
  "tenants_statuses",
  [
    %{id: "on", name: "On", description: "Signups and logins for this tenant are enabled"},
    %{id: "off", name: "Off", description: "Signups and logins for this are disabled"},
  ]
)
Repo.insert_all(
  Auth.Schemas.Tenant,
  [
    %{
      id: "wholesale",
      name: "Wholesale",
      status_id: "on",
      description: "Deny Designs and the Wholesale channel",
    }
  ]
)

I’ve tried using both the string table name as the 1st argument or the schema module, but the error is the same. When I inspect the table inside PostGreSQL, I see that it has not defined a default value:

Column    |              Type              | Collation | Nullable |        Default        | Storage  | Stats target | Description 
-------------+--------------------------------+-----------+----------+-----------------------+----------+--------------+-------------
id          | character varying(16)          |           | not null |                       | extended |              | 
status_id   | character varying(10)          |           | not null |                       | extended |              | 
name        | character varying(32)          |           | not null | ''::character varying | extended |              | 
description | text                           |           | not null | ''::text              | extended |              | 
inserted_at | timestamp(0) without time zone |           | not null |                       | plain    |              | 
updated_at  | timestamp(0) without time zone |           | not null |                       | plain    |              | 

I thought that using timestamps() would set up a default value for both of those columns, but clearly I’m missing something. Can anyone point out what I’m missing? Thanks!