Cannot add member interests via Mailchimp API

Working with the Mailchimp API (v3) for the first time from my NodeJS app. I am also using https://www.npmjs.com/package/mailchimp-api-v3 to make the calls.

I am struggling to create interests when I add a new user to a list. My call looks like this:

mailchimp.post('/lists/<listID>/members', {
  email_address : email,
  status : 'subscribed',
  merge_fields : {
    'FNAME': fname,
    'LNAME': lname
  },
  interests : {
    'apples' : true,
    'bananas' : false,
    'pears' : false
  }
})

but I keep getting invalid interest ID "apples".

What else do I need to do here?

via tommyd456