Polynomial evaluation over finite fields

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Hello, how do I make Sagemath understand that it should evaluate polynomials in a finite field?

Currently, this is how I'm declaring my polynomial ring.

R.<x>=GF(5)[]
f=x^2
f(5)
>>> 25

Am I not declaring my polynomial ring properly?

I want to make Sagemath understand that whenever it sees a variable $x$, it should treat the polynomial as one being declared over $GF(5)$. How do I make Sagamath output $f(5)=0$ instead?