No upstream_response_time in access log file

[jonefee]

i use nginx as reverse proxy server , and php-fpm as fast cgi upstream.
i found some access in nginx access  log file doesen't has  a 
"$upstream_response_time" value  but a  "-" character instead ,,,,  and also
has "$response_time" value.
why ?

for example:

71.213.141.240 - 31/Oct/2012:13:09:34 +0800 POST
/php/xyz/iface/?key=be7fc0cdf0afbfedff1e09ec6443823a&device_id=351870052329449&network=1&ua=LT18i&os=2.3.4&version=3.7&category_id=2&f_ps=10&s=5&ps=30&pn=1&pcat=2&up_tm=1351655451272
HTTP/1.1 499 0($body_bytes_sent) - Dalvik/1.4.0 (Linux; U; Android 2.3.4;
LT18i Build/4.0.2.A.0.62) 21($content_length) 2.448($request_time)
-($upstream_response_time) - - -

the response_time is 2.448,,,but no upstream_response_time,,,,  the http
response code is 499,,,,,dosen't mean that nginx did not finish the
"connection"  and php-fpm even has no chance to "see" the access ?

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,232482,232482#msg-232482