StopIteration when doing --sort virtual on small filesystem about btrfs-heatmap HOT 4 CLOSED

-# ./show_usage.py /mnt/tutorial
dev item devid 1 total bytes 6442450944 bytes used 3026583552
dev item devid 2 total bytes 6442450944 bytes used 1961230336
dev item devid 3 total bytes 6442450944 bytes used 3034972160
block group vaddr 20971520 length 8388608 flags SYSTEM|RAID1 used 16384 used_pct 0
    chunk vaddr 20971520 type SYSTEM|RAID1 length 8388608 num_stripes 2
        stripe devid 3 offset 1048576
        stripe devid 2 offset 1048576
block group vaddr 29360128 length 1073741824 flags METADATA|RAID1 used 140083200 used_pct 13
    chunk vaddr 29360128 type METADATA|RAID1 length 1073741824 num_stripes 2
        stripe devid 1 offset 20971520
        stripe devid 3 offset 9437184
block group vaddr 1103101952 length 1932656640 flags DATA|RAID0 used 1932591104 used_pct 100
    chunk vaddr 1103101952 type DATA|RAID0 length 1932656640 num_stripes 3
        stripe devid 2 offset 9437184
        stripe devid 1 offset 1094713344
        stripe devid 3 offset 1083179008
block group vaddr 3035758592 length 1962934272 flags DATA|RAID0 used 1962934272 used_pct 100
    chunk vaddr 3035758592 type DATA|RAID0 length 1962934272 num_stripes 3
        stripe devid 2 offset 653656064
        stripe devid 1 offset 1738932224
        stripe devid 3 offset 1727397888
block group vaddr 4998692864 length 1962934272 flags DATA|RAID0 used 955039744 used_pct 49
    chunk vaddr 4998692864 type DATA|RAID0 length 1962934272 num_stripes 3
        stripe devid 2 offset 1307967488
        stripe devid 1 offset 2393243648
        stripe devid 3 offset 2381709312

length = chunk.length * len(stripes)