What is the optimal margin when demand is inelastic?

The formula for the optimal margin of price over marginal cost is 1 / |e|, where e is the price elasticity of demand. This formula does not work when demand is |e| > 1. When |e| > 1 the demand is inelastic. The firm can always increase profit by increasing price when demand is inelastic and marginal cost is non-negative; the optimal price is infinitely high if elasticity and marginal cost are both constant.