Flint exception when factorizing a polynomial over finite field

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I get a Flint exception when trying to factorize the following homogeneous polynomial of degree 18 in x,y,z over the finite field with 3 elements:

sage: R.<x,y,z> = PolynomialRing(GF(3))

sage: P = x^18 + x^12y^6 + x^6y^12 + y^18 + x^12y^4z^2 - x^10y^6z^2 - x^6y^10z^2 + x^4y^12z^2 + x^12y^2z^4 - x^10y^4z^4 - x^4y^10z^4 + x^2y^12z^4 + x^12z^6 - x^10y^2z^6 + x^6y^6z^6 - x^2y^10z^6 + y^12z^6 - x^6y^2z^10 - x^4y^4z^10 - x^2y^6z^10 + x^6z^12 + x^4y^2z^12 + x^2y^4z^12 + y^6z^12 + z^18

sage: P.factor()

Flint exception (General error): lenP < 2 in n_poly_mod_invmod

What am I doing wrong here ?