Interest question in Bitmex ( No risk for the earning interest?)

[balllighting]

Free interest in Bitmex ?

Just asked in "trading" but maybe wrong. I think ask in here will be better!
https://bitcointalk.org/index.php?topic=1967548.msg19568512#msg19568512

I am a newbie in bitcoin, and I found that there are two type of contracts for BTCUSD in Bitmex, one is called XBTH17 and other is called XBTUSD.

And I found that if we short XBTUSD, there will be a lot of interest everyday.

So I wonder if I Long 1,000 XBTH17 contracts and short 1,000 XBTUSD contract.

Is this can earn free interest every day? When the price of BTC going up, XBTH17  will cover XBTUSD's loss and vice versa!

Is there any potential risk?