Can't unpack new gts firmware

[AsafMah]

Hey, I tried the tool on a gts firmware https://wearyour.tech/wp-content/uploads/2020/03/kestrelw_0.0.8.52_ft12.zip
and I get this error:

Unpacking .\kestrelw_myamazfit.ru_v12.ft
fontVersion 12
non-Latin: False
Latin:     False
last_block b'\x85\x88\x05\x00\xfc\xfe\x17\x1b\x1a\x01\x16\xff\x00\x01'
ptr=000000 85 88 05 00 fc fe 17 1b 1a 01 16 ff 00 01 img_data_addr=00000000+0x20 unicode=fefc
w: 23 h:27
dimensione_stimata 0x144
last_img_addr = 58885

pointer; 0x-0000e
Traceback (most recent call last):
  File "fonttool-gtr.py", line 347, in <module>
    unpackFont(args.filename, raw = args.raw, verbose = args.verbose)
  File "fonttool-gtr.py", line 156, in unpackFont
    value = file_content[pos]
IndexError: cannot fit 'int' into an index-sized integer