If there is a line in nginx log with empty user agent it fails with a message like this:

panic: Access log line 'xx.xx.xx.xx - [21/Apr/2014:06:25:10 +0000] "GET / HTTP/1.0" 200 15389 "-" ""' does not match given format '^(?P<remote_addr>[^ ]+) - [(?P<time_local>[^]]+)] "(?P[^ ]+) (?P[^ ]+) (?P[^"]+)" (?P[^ ]+) (?P[^ ]+) "(?P[^"]+)" "(?P[^"]+)"$'