!!! Aborted: subvolume is readonly

Good Morning, I apologize in advance if I am asking something silly but I am just trying to backup a simple btrfs subvolume to another FS as a stream and it seems that btrbk refuse to take a snapshot of a read-only subvolume?
I am probably missing something... any suggestions on how to deal with it?

my conf file:

```
root@HAL:/etc/btrbk# cat btrbk-ub.conf
raw_target_compress xz
raw_target_compress_threads 2
#raw_target_encrypt gpg
#gpg_keyring /etc/btrbk/gpg/pubring.gpg
#gpg_recipient btrbk@ffsb42.com
snapshot_dir btrbk_snapshots
volume /media/BIG
subvolume ub/dodo/190725-0610
snapshot_dir btrbk_snapshots
target raw /media/google-drive/btrbk/ub
# incremental no
```

my output:

```
root@HAL:/etc/btrbk# btrbk -c btrbk-ub.conf -vv -l debug run
btrbk command line client, version 0.27.1 (Sat Aug 3 11:48:41 2019)
Using configuration: btrbk-ub.conf
Initializing volume section: /media/BIG
Resolving btrfs mount point for: /media/BIG
### readlink -e /media/BIG
Command execution successful
Real path for "/media/BIG" is: /media/BIG
### cat /proc/self/mountinfo
Command execution successful
Btrfs mount point for "/media/BIG": /media/BIG (mount_source=/dev/sdb1, subvolid=5)
### btrfs subvolume list -a -c -u -q -R /media/BIG
Command execution successful
Parsed 640 total subvolumes for filesystem at: /media/BIG
### btrfs subvolume list -a -r /media/BIG
Command execution successful
Parsed 631 readonly subvolumes for filesystem at: /media/BIG
### btrfs subvolume show --rootid=5 /media/BIG
Command execution successful
Parsed 11 subvolume detail items: /media/BIG
found btrfs root: /media/BIG
Initializing subvolume section: /media/BIG/ub/dodo/190725-0610
Resolving btrfs mount point for: /media/BIG/ub/dodo/190725-0610
### readlink -e /media/BIG/ub/dodo/190725-0610
Command execution successful
Real path for "/media/BIG/ub/dodo/190725-0610" is: /media/BIG/ub/dodo/190725-0610
Btrfs mount point for "/media/BIG/ub/dodo/190725-0610": /media/BIG (mount_source=/dev/sdb1, subvolid=5)
**WARNING: Skipping subvolume "/media/BIG/ub/dodo/190725-0610": subvolume is readonly**
Completed within: 1s (Sat Aug 3 11:48:42 2019)
--------------------------------------------------------------------------------
Backup Summary (btrbk command line client, version 0.27.1)

Date: Sat Aug 3 11:48:41 2019
Config: btrbk-ub.conf

Legend:
=== up-to-date subvolume (source snapshot)
+++ created subvolume (source snapshot)
--- deleted subvolume
*** received subvolume (non-incremental)
>>> received subvolume (incremental)
--------------------------------------------------------------------------------
/media/BIG/ub/dodo/190725-0610
**!!! Aborted: subvolume is readonly**

NOTE: Some errors occurred, which may result in missing backups!
Please check warning and error messages above.

```