Keyesian Mathematics

On Crossfire today, Keyes said that slaves were taxed at a 100% rate. I would just like to point out the mathematical improbability of such a tax rate. Rate rate is computed by dividing tax paid by income (and taking it times 100 to expres as a percentage). However, when slave income is $0, the denominator is zero and the result is what mathematicians consider "undefined." When Keyes loses, perhaps he can get a job here.