text: Title: returns None
  Body: Please tell me why it returns None
  
  ```python
  from pybit import usdt_perpetual
  import time
  import pandas as pd
  import pandas_ta as ta
  import numpy as np
  
  print(ta.version)
  0.3.14b0
  
  session_unauth = usdt_perpetual.HTTP(endpoint="https://api.bybit.com")
  def klines():
    session=session_unauth.query_mark_price_kline(symbol="BTCUSDT", interval=60, limit=500, from_time=int(time.time()-60*60*120))
    session=session['result']
    df=pd.DataFrame(session)
    df=df.iloc[:,[6]]
    return df
  
  print(type(klines()))
  close = klines()
  m = ta.sma(close, 14)
  print(m)
  ```
  
  ```sh
  <class 'pandas.core.frame.DataFrame'>
  None
  ```