'NoneType' object has no attribute 'replace' when reading asd files

[YanCheng-go]

Traceback (most recent call last):
File "C:/Users/ChengY/.PyCharmCE2019.3/config/scratches/scratch.py", line 64, in
s = specdal.Spectrum(filepath=os.path.join(work_dir, date, file_name))
File "C:\Program Files (x86)\Python37_6\lib\site-packages\specdal\spectrum.py", line 55, in init
self.read(filepath, measure_type, verbose=verbose)
File "C:\Program Files (x86)\Python37_6\lib\site-packages\specdal\spectrum.py", line 79, in read
data, meta = read(filepath, verbose=verbose)
File "C:\Program Files (x86)\Python37_6\lib\site-packages\specdal\reader.py", line 18, in read
read_metadata, verbose)
File "C:\Program Files (x86)\Python37_6\lib\site-packages\specdal\reader.py", line 155, in read_asd
ref_column = tgt_column.replace('tgt', 'ref')
AttributeError: 'NoneType' object has no attribute 'replace'