cannot remove 'v3.0': Directory not empty”

But it is empty???

rails@snarf:~$ ls -la /var/www/robl.me/releases/20160323235726/tmp/cache/assets/sprockets/v3.0
total 40
drwxrwxr-x 2 rails rails 36864 Dec 16 15:11 .
drwxrwxr-x 3 rails rails  4096 Mar 23  2016 ..
rails@snarf:~$ rmdir /var/www/robl.me/releases/20160323235726/tmp/cache/assets/sprockets/v3.0
rmdir: failed to remove ‘/var/www/robl.me/releases/20160323235726/tmp/cache/assets/sprockets/v3.0’: Directory not empty