Was Stannis expected to pay the existing debts of Westeros?

I don't quite understand why the Iron Bank agreed to loan money to Stannis Baratheon to fund his army. If he lost, they were out of pocket for the money loaned, and if he won, they were out of pocket for the money they had previously loaned to Robert Baratheon/Westeros.

..Or were they? Was there an understanding that on winning, Stannis would repay not just his own debts, but those of Robert Baratheon/Westeros?