Sorting by time fields results in an error

[bapril]

Working query to verify setup

dnsdb-query$ ./dnsdb_query.py -l 4 -r *.bac.com/ns
;; bailiwick: com.
;; count: 1,275
;; first seen in zone file: 2013-01-12 17:16:08 -0000
;; last seen in zone file: 2016-07-20 16:15:34 -0000
bac.com. IN NS ns6.bac.com.
bac.com. IN NS ns7.bac.com.
bac.com. IN NS ns8.bac.com.
bac.com. IN NS ns10.bac.com.
bac.com. IN NS ns11.bac.com.
bac.com. IN NS ns12.bac.com.

;; bailiwick: com.
;; count: 364
;; first seen in zone file: 2012-01-14 17:13:33 -0000
;; last seen in zone file: 2013-01-11 17:16:41 -0000
bac.com. IN NS ns10.bac.com.
bac.com. IN NS ns11.bac.com.
bac.com. IN NS ns12.bac.com.
bac.com. IN NS ns1.bankofamerica.com.
bac.com. IN NS ns3.bankofamerica.com.
bac.com. IN NS ns4.bankofamerica.com.

;; bailiwick: com.
;; count: 630
;; first seen in zone file: 2010-04-24 16:12:21 -0000
;; last seen in zone file: 2012-01-13 17:14:40 -0000
bac.com. IN NS ns1.bankofamerica.com.
bac.com. IN NS ns3.bankofamerica.com.
bac.com. IN NS ns4.bankofamerica.com.

;; bailiwick: com.
;; count: 2
;; first seen: 2015-10-16 08:45:12 -0000
;; last seen: 2015-10-16 08:45:12 -0000
bac.com. IN NS ns6.bac.com.

Failure-mode A

dnsdb-query$ ./dnsdb_query.py -r *.bac.com/ns --sort time_last
dnsdb_query: invalid sort key "time_last". valid sort keys are bailiwick, count, rdata, rrname, rrtype, zone_time_first, zone_time_last

Failure-mode B

dnsdb-query$ ./dnsdb_query.py -r *.bac.com/ns --sort zone_time_last
Traceback (most recent call last):
File "./dnsdb_query.py", line 317, in
main()
File "./dnsdb_query.py", line 201, in f
return func(_args, *_kwargs)
File "./dnsdb_query.py", line 309, in main
results.sort(key=lambda r: r[options.sort], reverse=options.reverse)
File "./dnsdb_query.py", line 309, in
results.sort(key=lambda r: r[options.sort], reverse=options.reverse)
KeyError: 'zone_time_last'