Is the preimage of the generic point of the map $\mathbb{P}^n_R \to \operatorname{Spec}R$ open?

Question

Let $R$ be an integral domain and let $z$ be the generic point of $\operatorname{Spec}R$. Let $\pi: \mathbb{P}^n_R \to \operatorname{Spec}R$. I want to show that $\pi^{-1}(z)$ is open. Any comments and suggestions are appreciated. Thank you.