mobx-react-form: computed value is not an empty array when fields are undefined

hi, ran into an issue today when attempting to use del()

I have an array for a field

let fields = [
    'argh',
    'argh[]',
]

if i give argh the value [‘a’, ‘b’, ‘c’] and then attempt to do form.$(argh).del(1) the new value will be ['b', 'c'] (1-based index) instead of ['a', 'c'] (0-based index)

is this intentional? if so why?

Thanks