Issue applying Descarte's rule of signs to $y=x^2$.

Question

Descarte's rule of signs for $p(x)=x^n$ for $n>0$, says that there are zero roots in this case. However, we can clearly see that it has 1 root at $x=0$. Could somebody please explain why is the rule not applicable and what other cases one can't use the rule for? Thank you all.