I think that if you play with your money of course it's not a problem. But if you play with bonus money until the balance is 0 I think you will have a problem. If you reach 0 euro it will be considerered that you don't have any bonus money and you will not have a bonus in your account...but you will have bonus rounds and winnings will go to real money balance after. That's why casinos don't allow this, you must keep something in balance until opening...but not sure how it is at Unibet...it will be strange to let you do this :)