cannot cut utf-8 input file, but can cut gbk file

[l1t1]

D:\Python35-32>python -m thulac inputu.txt output.txt seg_only
Model loaded succeed
Traceback (most recent call last):
File "D:\Python35-32\lib\runpy.py", line 184, in run_module_as_main
"main", mod_spec)
File "D:\Python35-32\lib\runpy.py", line 85, in run_code
exec(code, run_globals)
File "D:\Python35-32\lib\site-packages\thulac_main.py", line 9, in
lac.cut_f(sys.argv[1], sys.argv[2])
File "D:\Python35-32\lib\site-packages\thulac_init.py", line 189, in cut_f
for line in input_f:
UnicodeDecodeError: 'gbk' codec can't decode byte 0xb4 in position 42: illegal multibyte sequence

D:\Python35-32>python -m thulac input.txt output.txt seg_only
Model loaded succeed
successfully cut file input.txt!