How to get gas for transaction?

I want use estimate tx.estimate_fee() in order to get gas . set this function like this :
but get this error when set gas =“auto” or None.
“error”: “int() argument must be a string, a bytes-like object or a real number, not ‘NoneType’”,

signer_opt = SignerOptions(
address=address, #terra16va7g7m6xa3mq7x5zmpquj28w5npsftep3hv37
)
tx_opt = CreateTxOptions(
msgs=[msg], memo=“send test”, gas_adjustment=1.5, fee_denoms=[“uluna”], gas_prices=gas_price + denom
, gas=‘auto’
)
estimate_fee = self.LCDClient.tx.estimate_fee([signer_opt], tx_opt)