How do I specify a btrfs subvolume?

[Nathan Dehnel]

This is extremely confusing:
https://guix.gnu.org/manual/en/html_node/Btrfs-file-system.html

I made my best attempt at creating and mounting a subvolume at
/media/offsite-backup:

  (file-systems
    (cons* (file-system
             (mount-point "/")
             (device
               (uuid "b2e0f449-0583-46fd-8c23-ad2e6cd661a1"
                     'btrfs))
             (type "btrfs"))
           (file-system
             (mount-point "/media/offsite-backup")
             (device
               (uuid "b2e0f449-0583-46fd-8c23-ad2e6cd661a1"
                     'btrfs))
             (type "btrfs")
             (options "subvol=offsite-backup,compress=zlib:9"))

It gives this error:

building 
/gnu/store/mrs1qbg619qk4n2lv8k377arclgzwqjq-install-bootloader.scm.drv...
guix system: bootloader successfully installed on '/dev/sda'
guix system: warning: exception caught while executing 'start' on
service 'file-system-/media/offsite-backup':
In procedure mount: mount "/dev/sda2" on "///media/offsite-backup": No
such file or directory
guix system: warning: some services could not be upgraded

But /media/offsite-backup exists.