How could I (painlessly) split or reverse "Last, First" within a record in Miller?

I have a tab-delimited file where one of the columns is in the format "LastName, FirstName". What I want to do is split that record out into two separate columns, last, and first, use cut or some other verb(s) on _that_, and output the result to JSON. I should add that I'm not married to JSON, and I know how to use other tools like [jq](https://github.com/stedolan/jq) , but it would be nice to get it in that format in one step. The syntax for the nest verb looks like it requires memorizing a lot of frankly non-memorable options, so I figured that there would be a simple DSL operation to do this job. Maybe that's not the case? Here's what I've tried. (Let's just forget about the extra space that's attached to Firstname right now, OK? I would use strip or ssub or something to get rid of that later.)

echo -e "last_first\nLastName, Firstname" \
  | mlr --t2j put '$o=splitnv($last_first,",")'

# result:
# { "last_first": "LastName, Firstname", "o": "(error)" }

# expected something like:
# { "last_first": "LastName, Firstname", "o": { 1: "LastName", 2: "Firstname" } }
#
# or:
# { "last_first": "LastName, Firstname", "o": [ "LastName", "Firstname" ] }

Why (error)? Is it not reasonable that assigning to $o as above would assign a new column o to the result of splitnv? Here's something else I tried that didn't work like I would've expected either:

echo -e "last_first\nLastName, Firstname" \
  | mlr -T nest --explode --values --across-fields --nested-fs , -f last_first

# result (no delimiter here, just one field, confirmed w/ 'cat -A')
# last_first
# LastName, Firstname

# expected:
# last_first_1last_first_2
# LastName, Firstname

**Edit**: The problem with the command above is I should've used --tsv, **not** -T, which is a synonym for --nidx --fs tab (numerically-indexed columns). Problem is, Miller doesn't produce an error message when it's obviously wrong to ask for named columns in that case, which might be a mis-feature; see [issue #233](https://github.com/johnkerl/miller/issues/233) . Any insight would be appreciated.