Issue with validation in array of references in a document field definition.

{
  name: 'types',
  title: 'Product type',
  description: 'You can select multiple types for compound products.',
  type: 'array',
  of: [
    defineArrayMember({
      type: 'reference',
      to: [{type: 'productType'}],
      options: {disableNew: true},
    }),
  ],
  validation: (Rule) => Rule.required().unique().error('Cannot add same product type twice'),
},