When theta is equal to zero, you have a zero in the denominator. And so what we need to do in order for this f of x, or in order for this to be the same thing, we have to say theta cannot be equal to zero. But now let's think about the limit again. Essentially what we want to do is we want to find the limit as theta approaches zero of f of x. And we can't just do direct substitution into, if we really take this seriously, because we're going to like, oh, well, if I try to put zero here, it says theta cannot be equal to zero. f of x is not defined at zero. This expression is defined at zero, but this tells me, well, I really shouldn't apply zero to this function.