"Insufficient funds" error for short selling on Kraken with CCXT

Question

I'm attempting to short sell with leverage on Kraken via the CCXT API with the following example code:

exchange = ccxt.kraken({
'apiKey': '****',
'secret': '****'

})

amount = ((exchange.fetch_balance()['USD']['total'])-10) / exchange.fetch_ticker('BTC/USD)[
                'last']
exchange.create_order('BTC/USD', 'market', 'buy', amount, {'leverage': 2})

But it gives the error

ccxt.base.errors.InsufficientFunds: kraken {"error":["EOrder:Insufficient funds"]}

I assume its because it isn't "borrowing" the funds as I expect it to, but I'm unsure why. I am unable to choose a specific amount each time because I'm actually running the code over 4 coins and it picks the one with the most movement and creates an order accordingly, thus to always ask it to buy/sell 0.001 isn't logical. I have attempted to ask it to buy/sell the maximum available as governed by my current balance as given by the "amount" variable. The -10 term is just a little buffer to account for some slippage.