Find extrema $\displaystyle (x^2-8)^{2/3}$

Answers are $\displaystyle \pm 2\sqrt{2} $ and 0 for x values,

Local minimas are $\displaystyle (\pm 2\sqrt{2},0)$

Which makes complete sense if you look at the graph, however algebraically I solve for local maxima (0,4) which makes no sense to me as its undefined on the graph, why does my book say it counts as a local maxima if its "undefined"?