diff --git a/freqtrade/data/converter/converter.py b/freqtrade/data/converter/converter.py
index 126dbe76b..a80b00319 100644
--- a/freqtrade/data/converter/converter.py
+++ b/freqtrade/data/converter/converter.py
@@ -62,7 +62,7 @@ def _init_dataframe_with_trades_columns(dataframe: DataFrame):
     dataframe['stacked_imbalances_ask'] = np.nan
 
 
-def _convert_timeframe_to_pandas_frequency(timeframe: str) -> str:
+def _convert_timeframe_to_pandas_frequency(timeframe: str):
     # convert timeframe to format usable by pandas
     from freqtrade.exchange import timeframe_to_minutes
     timeframe_minutes = timeframe_to_minutes(timeframe)