btrfs-image, bogus ERROR: either extent tree is corrupted or deprecated extent ref format

[cmurf]

kernel 5.8.1
btrfs-progs 5.7

# btrfs-image -c1 -t4 -ss /dev/whatever
...
WARNING: cannot find a hash collision for 'Furr', generating garbage, it won't match indexes
WARNING: cannot find a hash collision for 'Logs', generating garbage, it won't match indexes
ERROR: either extent tree is corrupted or deprecated extent ref format
ERROR: create failed: -5
# btrfs check /dev/whatever
Opening filesystem to check...
Checking filesystem on /dev/mapper/sdc
UUID: e057b060-5087-4616-a954-ed79c00b96dc
[1/7] checking root items
[2/7] checking extents
[3/7] checking free space tree
[4/7] checking fs roots
[5/7] checking only csums items (without verifying data)
[6/7] checking root refs
[7/7] checking quota groups skipped (not enabled on this FS)
found 862654930944 bytes used, no error found
total csum bytes: 837779688
total tree bytes: 4768530432
total fs tree bytes: 3583033344
total extent tree bytes: 295501824
btree space waste bytes: 828122451
file data blocks allocated: 18932597014528
 referenced 5382926192640