Add a new column and also index it using knex?

I have a simple knext migration to add a new column:

import * as Knex from "knex";

export async function up(knex: Knex): Promise<void> {
  await knex.schema.alterTable("bins", (table) => {
    table.bigInteger("parent_lsn").defaultTo(null).after("lsn").index();
  });
}

export async function down(knex: Knex): Promise<void> {
  await knex.schema.alterTable("bins", (table) => {
    table.dropColumns("parent_lsn");
  });
}

I tried adding the .index() in there, but it's not seeming to reflect as an indexed column in the database. Not sure where I'm going wrong here?