(logrotate) gzip: stdin: file size changed while zipping

rotating log /var/log/upstart/php5-fpm.log, log->rotateCount is 7
dateext suffix '-20150608'
glob pattern '-[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]'
renaming /var/log/upstart/php5-fpm.log.7.gz to /var/log/upstart/php5-fpm.log.8.gz (rotatecount 7, logstart 1, i 7),
renaming /var/log/upstart/php5-fpm.log.6.gz to /var/log/upstart/php5-fpm.log.7.gz (rotatecount 7, logstart 1, i 6),
renaming /var/log/upstart/php5-fpm.log.5.gz to /var/log/upstart/php5-fpm.log.6.gz (rotatecount 7, logstart 1, i 5),
renaming /var/log/upstart/php5-fpm.log.4.gz to /var/log/upstart/php5-fpm.log.5.gz (rotatecount 7, logstart 1, i 4),
renaming /var/log/upstart/php5-fpm.log.3.gz to /var/log/upstart/php5-fpm.log.4.gz (rotatecount 7, logstart 1, i 3),
renaming /var/log/upstart/php5-fpm.log.2.gz to /var/log/upstart/php5-fpm.log.3.gz (rotatecount 7, logstart 1, i 2),
renaming /var/log/upstart/php5-fpm.log.1.gz to /var/log/upstart/php5-fpm.log.2.gz (rotatecount 7, logstart 1, i 1),
renaming /var/log/upstart/php5-fpm.log.0.gz to /var/log/upstart/php5-fpm.log.1.gz (rotatecount 7, logstart 1, i 0),
old log /var/log/upstart/php5-fpm.log.0.gz does not exist
renaming /var/log/upstart/php5-fpm.log to /var/log/upstart/php5-fpm.log.1
compressing log with: /bin/gzip
switching uid to 0 and gid to 103
gzip: stdin: file size changed while zipping
removing old log /var/log/upstart/php5-fpm.log.8.gz

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: php5-fpm 5.5.9+dfsg-1ubuntu4.9
ProcVersionSignature: Ubuntu 3.13.0-51.84-generic 3.13.11-ckt18
Uname: Linux 3.13.0-51-generic i686
ApportVersion: 2.14.1-0ubuntu3.11
Architecture: i386
Date: Mon Jun 8 11:19:22 2015
InstallationDate: Installed on 2014-01-21 (502 days ago)
InstallationMedia: Ubuntu-Server 12.04.3 LTS "Precise Pangolin" - Release i386 (20130820.2)
SourcePackage: php5
UpgradeStatus: Upgraded to trusty on 2014-04-18 (416 days ago)
mtime.conffile..etc.php5.fpm.php.fpm.conf: 2014-11-25T12:45:00.338857