Array Element in x-faker Doesn't Work as Expected

[AddisonSnyder]

The goal is to choose a random element using x-faker, as below:

  unitOfPurchase:
    type: string
    description: |
      FileMan Field Label: 'UNIT OF PURCHASE'  FileMan Field Number: '1.5'
      This is the unit of purchase for this item.
    title: UNIT OF PURCHASE
    x-faker:
      helpers.arrayElement: [ 'yard', 'foot' ]

All it gives is a random letter from the word 'yard'. Is this the proper way to use the x-faker method for getting a random array element?