I use the if trick to get timestamped log names:

if ($time_iso8601 ~ "^(\d{4})-(\d{2})") {
set $year $1;
set $month $2;
}

access_log .../access-$year-$month.log combined;

However, with nginx/1.4.6 (Ubuntu) I see that for invalid HTTP requests $year and $month are not set. For example, with the above config after issuing from a shell:

~> printf foo | nc host 80

I see in the error log for nginx:

2014/11/29 21:28:57 [warn] 8#0: *18 using uninitialized "year" variable while logging request, client: 172.17.42.1, server: localhost, request: "foo"
2014/11/29 21:28:57 [warn] 8#0: *18 using uninitialized "month" variable while logging request, client: 172.17.42.1, server: localhost, request: "foo"

That leads for the log file named .access--.log

Is it because $time_iso8601 is only set for valid request? If so, is this just a bug or a feature?