Rollup with show_original function using Notion API

Question

I'm trying to retrieve a rollup property value that's of type "select", like so:

const rollupName = 'foo';
const prop = await notion.pages.properties.retrieve({
  page_id: page.id,
  property_id: page.properties[rollupName].id,
});

page.properties contains the rollup property:

foo: {
    id: 'Rv%5Cn',
    type: 'rollup',
    rollup: { type: 'array', array: [], function: 'show_original' }
  },

I'm getting the following error:

{"object":"error","status":400,"code":"validation_error","message":"Invalid relations found in rollup."}

How can I get the select value?