# Buggy code
def get_sell_rate(self, pair: str, refresh: bool) -> float:
    """
    Get sell rate - either using get-ticker bid or first bid based on orderbook
    The orderbook portion is only used for rpc messaging, which would otherwise fail
    for BitMex (has no bid/ask in fetch_ticker)
    or remain static in any other case since it's not updating.
    :return: Bid rate
    """
    config_ask_strategy = self.config.get("ask_strategy", {})
    if config_ask_strategy.get("use_order_book", False):
        logger.debug("Using order book to get sell rate")

        order_book = self.exchange.get_order_book(pair, 1)
        rate = order_book["bids"][0][0]

    else:
        rate = self.exchange.fetch_ticker(pair, refresh)["bid"]
    return rate


# Traceback (Exception)
2020-02-20 22:47:28,025 - freqtrade - ERROR - Fatal exception!
Traceback (most recent call last):
File "/opt/tradebot/freqtrade/.env/lib/python3.7/site-packages/talib/__init__.py", line 20, in wrapper
for arg in chain(args, kwargs.values())
StopIteration

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/freqtrade/freqtrade/main.py", line 36, in main
return_code = args['func'](args)
File "/freqtrade/freqtrade/commands/optimize_commands.py", line 76, in start_hyperopt
hyperopt.start()
File "/freqtrade/freqtrade/optimize/hyperopt.py", line 488, in start
preprocessed = self.backtesting.strategy.tickerdata_to_dataframe(data)
File "/freqtrade/freqtrade/strategy/interface.py", line 448, in tickerdata_to_dataframe
for pair, pair_data in tickerdata.items()}
File "/freqtrade/freqtrade/strategy/interface.py", line 448, in <dictcomp>
for pair, pair_data in tickerdata.items()}
File "/freqtrade/freqtrade/strategy/interface.py", line 464, in advise_indicators
return self.populate_indicators(dataframe, metadata)
File "/freqtrade/user_data/strategies/MyDev.py", line 205, in populate_indicators
dataframe['mfi'] = ta.MFI(dataframe)
File "talib/_abstract.pxi", line 352, in talib._ta_lib.Function.__call__
File "talib/_abstract.pxi", line 383, in talib._ta_lib.Function.__call_function
File "/opt/tradebot/freqtrade/.env/lib/python3.7/site-packages/talib/__init__.py", line 24, in wrapper
return func(*args, **kwargs)
File "talib/_func.pxi", line 8454, in talib._ta_lib.MFI
Exception: volume is not double

# Fixed code
def get_sell_rate(self, pair: str, refresh: bool) -> float:
    """
    Get sell rate - either using get-ticker bid or first bid based on orderbook
    The orderbook portion is only used for rpc messaging, which would otherwise fail
    for BitMex (has no bid/ask in fetch_ticker)
    or remain static in any other case since it's not updating.
    :param pair: Pair to get rate for
    :param refresh: allow cached data
    :return: Bid rate
    """
    if not refresh:
        rate = self._sell_rate_cache.get(pair)
        # Check if cache has been invalidated
        if rate:
            logger.info(f"Using cached sell rate for {pair}.")
            return rate

    config_ask_strategy = self.config.get("ask_strategy", {})
    if config_ask_strategy.get("use_order_book", False):
        logger.debug("Using order book to get sell rate")

        order_book = self.exchange.get_order_book(pair, 1)
        rate = order_book["bids"][0][0]

    else:
        rate = self.exchange.fetch_ticker(pair)["bid"]
    self._sell_rate_cache[pair] = rate
    return rate