csum errors on negative root id?

[Ross Vandegrift <ross@kallisti.us>]

Hello,

I recently saw these in my dmesg, on 4.19.160:
  BTRFS warning (device dm-20): csum failed root -9 ino 1303 off 0 csum 0x47abffd5 expected csum 0xd5f0d335 mirror 1
  BTRFS warning (device dm-20): csum failed root -9 ino 1303 off 0 csum 0x47abffd5 expected csum 0xd5f0d335 mirror 1

I'm confused by a few things here:
- does the negative root id mean something, or is this a bug?
- inode 1303 doesn't seem to exist.
- This is a standalone drive, is this about a dup metadata block?

Details on the fs:
$ sudo btrfs fi show /mnt/storage
Label: none  uuid: 6a2038fb-a9b4-4720-a441-c084610e4295
        Total devices 1 FS bytes used 4.66TiB
        devid    1 size 7.28TiB used 5.72TiB path /dev/mapper/storage

$ sudo btrfs fi df /mnt/storage
Data, single: total=4.68TiB, used=4.66TiB
System, DUP: total=32.00MiB, used=768.00KiB
Metadata, DUP: total=536.45GiB, used=4.95GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
$ sudo btrfs inspect-internal subvolid-resolve -- -9 /mnt/storage
ERROR: -9: negative value is invalid.
$ sudo btrfs inspect-internal inode-resolve 1303 /mnt/storage
ERROR: ino paths ioctl: No such file or directory

Please keep me CCed, thanks!

Ross