error while trying to unpack

$ ./unpack_bootimg.py --boot_img ~/hadk/out/target/product/fortuna3g/hybris-boot.img --out ../unpacked/
Traceback (most recent call last):
  File "./unpack_bootimg.py", line 563, in <module>
    main()
  File "./unpack_bootimg.py", line 558, in main
    info = unpack_bootimg(args.boot_img, args.out)
  File "./unpack_bootimg.py", line 484, in unpack_bootimg
    info = unpack_boot_image(image_file, output_dir)
  File "./unpack_bootimg.py", line 221, in unpack_boot_image
    info.cmdline = cstr(unpack('1536s',
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x82 in position 0: invalid start byte