How can I delete a apt-btrfs-snapshot when the error is "Directory not empty?"

How can I delete a apt-btrfs-snapshot when the error is "Directory not
empty?"

There is one snapshot created by apt-btrfs-snapshot that won't delete
Delete subvolume '/tmp/foo/@apt-snapshot-2013-04-XX_1X:3X:XX'
ERROR: cannot delete '/tmp/foo/@apt-snapshot-2013-04-XX_1X:3X:XX' -
Directory not empty
How can I delete it?