Exception for 2-ancestry mixing, zero discriminant

Math

\[\sqrt[3]{\frac{g}{2 \cdot a}}\]

C

double f(double g, double a) {
        double r8166 = g;
        double r8167 = 2.0;
        double r8168 = a;
        double r8169 = r8167 * r8168;
        double r8170 = r8166 / r8169;
        double r8171 = cbrt(r8170);
        return r8171;
}

Reproduce

Please include this information when filing a bug report:

herbie shell --seed 2019191 
(FPCore (g a)
  :name "2-ancestry mixing, zero discriminant"
  (cbrt (/ g (* 2.0 a))))

Backtrace

get-representation: Unknown representation #f		L	C
loop	/data/pavpan/nightlies/herbie/interface2/src/points.rkt	122	4
prepare-points	/data/pavpan/nightlies/herbie/interface2/src/points.rkt	146	0
setup-prog!34	/data/pavpan/nightlies/herbie/interface2/src/mainloop.rkt	67	0
run-improve43	/data/pavpan/nightlies/herbie/interface2/src/mainloop.rkt	339	0
(unnamed)	/opt/racket-7.0/collects/racket/private/more-scheme.rkt	261	28
run	/opt/racket-7.0/share/pkgs/profile-lib/main.rkt	39	2
profile-thunk16	/opt/racket-7.0/share/pkgs/profile-lib/main.rkt	9	0
(unnamed)	/opt/racket-7.0/collects/racket/private/more-scheme.rkt	261	28