Why Nifty 50 symbol "NSE:NIFTY50-INDEX" always return empty response on fyersSocket.subscribe() (Fyers V2)?

I am trying to fetch market data for Nifty 50 index using fyers websocket but it always return empty response as below. The problem is same for BANK NIFTY index also

Custom []
pong received
Custom []
Custom []

Below is the code I used:

def custom_message(self):
    print ("Custom " + str(self.response))    

ws.FyersSocket.websocket_data = custom_message
acctoken = client_id+':'+accessToken
data_type ="symbolData" 
symbol =  ["NSE:NIFTY50-INDEX"] 
fyersSocket = ws.FyersSocket(access_token=acctoken, data_type=data_type,symbol=symbol)
fyersSocket.subscribe()

But it works for the respective strike price symbol like: "NSE:NIFTY21SEP17850CE"