mysqldump in postrotate fails with automatic rotate

Question

I have this simple logrotate script

/var/backups/mysql/wordpress.sql.gz {
    rotate 7
    daily
    sharedscripts
    missingok
    nocompress
    postrotate
        /usr/bin/mysqldump wordpress | /usr/bin/gzip >/var/backups/mysql/wordpress.sql.gz
    endscript
}

This works fine when I run it by hand with logrotate -f script but not with the automatic rotate. I get an empty wordpress.sql so I assume it's the mysqldump that fails but I don't know why.