AttributeError: 'NoneType' object has no attribute 'loc'

[benglesII]

2021-04-21 00:21:59,016 - freqtrade - ERROR - Fatal exception!
Traceback (most recent call last):
File "/freqtrade/freqtrade/main.py", line 37, in main
return_code = args'func'
File "/freqtrade/freqtrade/commands/optimize_commands.py", line 86, in start_hyperopt
hyperopt.start()
File "/freqtrade/freqtrade/optimize/hyperopt.py", line 382, in start
preprocessed[pair] = trim_dataframe(df, timerange)
File "/freqtrade/freqtrade/data/converter.py", line 128, in trim_dataframe
df = df.loc[df[df_date_col] >= start, :]
AttributeError: 'NoneType' object has no attribute 'loc'

[benglesII]

missing return dataframe