infinity - Why does Michio Kaku say that $frac{1}{0} = infty$?

Why does Michio Kaku say that $\frac{1}{0} = \infty$?

http://youtu.be/AJ4zlvqOtE8?t=4m43s

Instead of $\frac{1}{0}$ that's not defined, so we don't know.