IndexError on ADX

[flaviovs]

Series length = 17, window = 14:

from io import StringIO

import pandas as pd
from ta.trend import adx

DATA = '''
DATE           O     H     L     C
2019-12-05  0.60  0.62  0.60  0.62
2019-12-06  0.61  0.61  0.61  0.61
2019-12-09  0.61  0.61  0.61  0.61
2019-12-10  0.60  0.60  0.60  0.60
2019-12-11  0.60  0.60  0.58  0.58
2019-12-12  0.59  0.59  0.58  0.58
2019-12-13  0.56  0.58  0.56  0.58
2019-12-16  0.55  0.57  0.55  0.56
2019-12-17  0.57  0.58  0.56  0.58
2019-12-18  0.59  0.59  0.58  0.58
2019-12-19  0.55  0.57  0.52  0.52
2019-12-20  0.51  0.56  0.51  0.53
2019-12-23  0.54  0.54  0.54  0.54
2019-12-24  0.53  0.57  0.53  0.57
2019-12-27  0.60  0.60  0.55  0.60
2019-12-30  0.62  0.62  0.60  0.60
2019-12-31  0.60  0.64  0.59  0.64
'''

df = pd.read_csv(StringIO(DATA), sep=' ', index_col='DATE', skipinitialspace=True, parse_dates=['DATE'])

print(len(df))
# '17'

adx(high=df['H'], low=df['L'], close=df['C'], window=14)

# ~/.venvs/lib/python3.7/site-packages/ta/trend.py:768: RuntimeWarning: invalid value encountered in double_scalars
#   dip[i] = 100 * (self._dip[i] / self._trs[i])
# ~/.venvs/lib/python3.7/site-packages/ta/trend.py:772: RuntimeWarning: invalid value encountered in double_scalars
#   din[i] = 100 * (self._din[i] / self._trs[i])
# Traceback (most recent call last):
#   File "test.py", line 31, in <module>
#     adx(df['H'], df['L'], df['C'], window=14)
#   File "~/.venvs/lib/python3.7/site-packages/ta/trend.#py", line 1251, in adx
#     high=high, low=low, close=close, window=window, fillna=fillna
#   File "~/.venvs/lib/python3.7/site-packages/ta/trend.py", line 777, in adx
#     adx_series[self._window] = directional_index[0 : self._window].mean()
# IndexError: index 14 is out of bounds for axis 0 with size 4

Issue stops with window set to 8 or less (no exception is raised at least, didn't check calculation), although the two warnings are still issued.