Logrotate of lighttpd logs failing due to directory permissions

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here is my configuration file /etc/logrotate.d/lighttpd after my changes

/var/log/lighttpd/*.log {
su www-data www-data
# add su line above; iefbr14
weekly
missingok
rotate 12
compress
delaycompress
notifempty
sharedscripts
postrotate
if [ -x /usr/sbin/invoke-rc.d ]; then
invoke-rc.d lighttpd reopen-logs > /dev/null 2>&1;
else
/etc/init.d/lighttpd reopen-logs > /dev/null 2>&1;
fi;
endscript
}

 (sorry about the indenting; the [code] tag is munging it :( )