Finding K such that this function has critical points

Determine the conditions on parameter $\displaystyle k$ such that the function $\displaystyle \displaystyle f(x)=\frac{2x+4}{x^2-k^2}$ will have critical points.

Critical points are defined as where $\displaystyle f'(x)=0$ or when $\displaystyle f'(x)$ does not exist. I was thinking it would be $\displaystyle k>0$ but the back of the book has the answer as $\displaystyle k$ being between $\displaystyle -2$ and $\displaystyle 2$.

Was my reasoning correct or am I missing something? I'm not sure why $\displaystyle k$ has to be between $\displaystyle -2$ and $\displaystyle 2$.