RethinkDB branch / do + "Argument 1 to deleteAt may not be `undefined`." - rethinkdb

Question

I am trying to write some somewhat complicated ReQL that deletes a single value from an array in a field, or deletes the record entirely if that value is the only one in the array.
I thought I had worked it out using branch/do/offsetAt, but then I ran into this "Argument 1 to deleteAt may not be undefined." error and I'm at a loss as to what the fix is.
r.branch(
r.db('db').table('table').get('uuid').eq(null),
{"deleted": 0, "errors": 0, "inserted": 0, "replaced": 0, "skipped": 0, "unchanged": 1},
r.db('db').table('table').get('uuid').getField('array_field').eq(['value']),
r.db('db').table('table').get('uuid').delete(),
r.do(
r.db('db').table('table').get('uuid').getField('array_field').offsetsOf('value'),
function(index) {
return r.branch(
index.eq([]),
{"deleted": 0, "errors": 0, "inserted": 0, "replaced": 0, "skipped": 0, "unchanged": 1},
r.db('db').table('table').get('uuid').update({
"array_field": r.row('array_field').deleteAt(index[0])
})
)
}
)
)
Also, as a side question, is there a more efficient way to do this without constantly fetching the record over and over calling .get()

Answer 1

So it turns out the issue here was in thinking I could use [] on the index.
Switching to .deleteAt(index.nth(0)) at line 16 solved the problem, or at least changed the error to "Cannot use r.row in nested queries"
So I rewrote the code to also solve my secondary issue, of constantly fetching the data using .get()
My new code is now
r.do(
r.db('db').table('table').get('uuid'),
function(record) {
return r.branch(
record.eq(null),
{"deleted": 0, "errors": 0, "inserted": 0, "replaced": 0, "skipped": 0, "unchanged": 1, "result": "no record"},
record.getField('array_field').eq(['array_value']),
r.db('db').table('table').get('uuid').delete(),
r.do(
record.getField('array_field').offsetsOf('array_value'),
function(index) {
return r.branch(
index.eq([ ]),
{"deleted": 0, "errors": 0, "inserted": 0, "replaced": 0, "skipped": 0, "unchanged": 1, "result": "no index"},
r.db('db').table('table').get('uuid').update({
"array_field": record.getField('array_field').deleteAt(index.nth(0))
})
)
}
)
)
}
)