The derivative of (x) = x 2 is zero at x = 0, but is not a constant function. Doesnt this contradict the corollary

The derivative of ƒ(x) = x² is zero at x = 0, but ƒ is not a constant function. Doesn’t this contradict the corollary of the Mean Value Theorem that says that functions with zero derivatives are constant? Give reasons for your answer.