Inner operation without the variables property produce an Uncaught TypeError: t.map is not a function

[MurzNN]

I need to build a query with nested operations, like this:

query ($id: Int) {
  thought (id: $id) {
    id,
    name,
    thought,
    details {
      image,
      description
    }
  }
}

So, I composed this JS code to make it:

const query = gql.query({
    operation: 'thought',
    variables: { id: 1 },
    fields: [
        'id',
        'name',
        'thought',
        {
            operation: 'details',
            fields: [
                'image',
                'description',
            ],
        },
    ]
})

But it produces an error:

Uncaught TypeError: t.map is not a function

But if I add an empty variables: {} property, it starts working, here is the example:

const query = gql.query({
    operation: 'thought',
    variables: { id: 1 },
    fields: [
        'id',
        'name',
        'thought',
        {
            operation: 'details',
            variables: {},
            fields: [
                'image',
                'description',
            ],
        },
    ]
})

So, is it a bug, or intended behavior? If intended, could you please describe the reasons?