pyserum: invalid public key input:',

I was implementing a buy function using pyserum and solana using the following code.

        payer=sender_keypair.public_key,
        owner=Account(sender_keypair),
        side=Side.BUY,
        order_type=OrderType.LIMIT,
        limit_price=0.5,
        max_quantity=abs(1),
        opts=TxOpts(skip_preflight=True)
    )

The following error occurs:

ValueError: ('invalid public key input:', '<solana.account.Account object at 0x7fac2a7c2cd0>')