Bitcoin futures trading made their debut at the Cboe Global Markets Inc. CBOE, on Dec. 10, and although the cash-settled, January contract US:XBTF8  has been mostly trending lower, it maintains a sizable spread between levels at the Cboe and other indexes. Such spreads are unusual in mature assets, with futures contracts tending to trade mostly in line with their underlying asset.

As of Wednesday afternoon, the spread between January bitcoin futures, at about $17,, was about $1,, with the quote for bitcoin BTCUSD, on data-and-research site conwaytransport.com.au at $16,, with prices broadly moving sharply lower. Bitcoin has gained $1, in alone. That compares with a nearly 25% year-to-date gain for the Dow Jones Industrial Average DJIA, and a roughly 20% rise so far in for the S&P index SPX, .

Interactive Brokers, however, will require a hefty fee for such short bets, with margin of $40, per contract for a short sale. Margin on long positions at the online platform is $9, Margin refers to the amount of money a trader must initially pony up as collateral when taking a futures position.

Interactive Broker’s steep margin requirements for short bets are likely because bitcoin has been among the most volatile assets, notably swinging as much as 20% or more within minutes and hours. Short bets, where an investor borrows an asset to return it at a lower price and collect a profit, can deliver infinite amounts of pain to an investor taking the wrong side of such a trade.

Hungarian-born Peterffy has been critical of the Cboe’s move to trade bitcoin futures, saying the wild trading swings of bitcoin could imperil both the trading platform and the investor who is shorting. The ability to place a short bet without having to first borrow the underlying security is one of the appeals of a futures market.

Read: Bitcoin futures: Here’s what you need to know

The CME Group CME,  plans on kicking off bitcoin futures trading Dec.

On the Cboe, the margin requirement for bitcoin futures trades 44% of the daily settlement price, while the CME will require bitcoin traders to put up a 35% margin.