Cash has no coupon, so it has negative coupon as soon as you switch your analysis to real terms. In fact, in the context of the fundamental theorem of asset pr

Cash has no coupon, so it has negative coupon as soon as you switch your analysis to real terms. In fact, in the context of the fundamental theorem of asset pricing (every asset discounted by a suitable numeraire is a martingale under the probability measure induced by that numeraire), cash isn’t even an asset (instead, the interest bearing bank account is one).